After a mysterious incident where both my brakes took on air after the big freeze (still no idea why) I bled them using the magura kit.

Now the rear is fine, but the front seems to suffer from some form of pump up; when I first grab the brake it comes within 1.5 cm of the bar, subsequent pumps bring the lever only as far back as 2.5/3cm from the bar. A much more normal distance. Leave it alone for a bit, and its back at 1.5 cm.

The brake lever feels hard when it comes to a stop, so I am at a loss. Any idea what's going on?

These brakes seem to be a little tricky to get fully bled. You've probably still got a small air pocket in the reservoir. As the bike's moved around the passageway to the master cylinder (lever) piston will be uncovered occasionally, letting a little air in. When you're bleeding and adding fluid through the port in the top cap you need to keep going past the point where the lever feel is firm and carry on until no more bubbles come out of the port. Make sure you do this with unworn pads though or next time you try to fit new ones you won't be able to push the pistons in the caliper back far enough

Aside, strictly speaking 'pump up' is where the brakes lose lever travel up to the point where they lock on, usually caused by excessive heat causing the fluid to expand.

Not quite sure what you mean by carrying on to add fluid past the point where the lever feel is firm. Following the instructions the fluid is added at the caliper end and the lever is actuated to flush air bubbles into the syringe on the top cap, but at no point in the process did the lever get firm. I assume You're bleeding these brakes in a different way?

I usually bleed with pads out and a wedge in place, as when I put the syringe onto the caliper (and again when it comes out) fluid comes out. I have never managed to do it with pads in place without contaminating a set of pads. sadly.

Noted re pump up! Thought I was using wrong terminology but didn't know the correct term.

sounds like you still have air in the system.
and by the sounds of it – you're doing the bleed correctly as per Magura instructions. Until you close of the system the lever will not be firm. Pumping the lever is just to help expel any trapped air.

try bleeding again?

and then if that doesn't work, try chatting to the support at Magura UK who are superb.

Connecting the syringe to the caliper is quite a messy way of doing the job so I usually only use that method if I am refilling a brake from completely empty. For dealing with just a small amount of trapped air I'll use a syringe with a hypodermic needle on it to inject fluid through the bleed port on the master cylinder. It might take a little longer but not much when you factor in cleaning up time. Pumping the lever is essential to dislodge air bubbles and ensure the fluid reaches all the cavities. Tilt the bike or lever to ensure the bleed port is the highest point of the system while you're doing it.

Interesting! I agree attaching anything to the bleed port on the caliper is messy and annoying, if I can do it without touching the caliper I will be a very happy chappy.

The magura syringe fits nicely onto the bleed port without a needle, but you have to stuff it in a bit. Can you do it this way? I'm not quite sure how a thin hypo needle would help, as wouldn't the fluid just come out from the gap the needle leaves?

So..:
Rotate lever assembly so master cylinder is level
pop off bleed port on cylinder
have some fluid in the syringe ready
put the syringe onto the bleed port

Now I am unsure as to the process:

Push some fluid into the system?
Draw the plunger upwards to pull bubbles into the syringe?
Pump the lever while pulling on the plunger?
Repeat, 5 times
With bubbles now all at the top of the syringe, push the plunger and push as much fluid as possible back into the system?

Take syringe off and put bleed port cap back on…

Done!

any tips or adjustments to that workflow?

I have just solved this problem on my magura's (Louise FR), when I say just I mean at 8:30 tonight.!!

I had all the same problems as you with the leaver hitting the bar it was that bad, this was after I found a hole in the hose. I bled the brakes so many times the rattle and the dummy came out the pram!!

So my fix is this. Bleed the brakes in the normal Magura way.
When done put the reservoir cap on then seal the bleed port on the calliper as normal then clean.

I put my new pads in and pumped the brake and got the same result, so I took the reservoir cap off again & moved the leaver all the way out using the adjustment screw. (Don't know if the Julies has this)

Then slowly squeeze the leaver until it wont go any further, when I released the leaver it came back very slowly but before it returned all the way out I squeezed it again. I repeated this 20+ times but eventually the the feel came back!!!
I put the brake back together and nipped up the road it is now as good as new!

Happy days 😀 hope this works for you.

Sweet, a combination of advice worked.. thanks all.

Wasn't up for rebleeding so whipped the top cap off the master cylinder, attached the syringe straight into the hole (with fluid in the barrel), and pushed some fluid into the system. Pumped the lever a bit and then pulled the plunger back up (bit further up than it was), and a shed load of bubbles streamed out. Did this a few times and eventually the bubbles stopped.

What made this work over the magura method was not having a syringe attached on the calliper, allowing the negative pressure (correct?) created by pulling the plunger up to draw the air to the top, instead of simply drawing the fluid in from the syringe at the caliper end.

This step is missing from the magura bleed kit, my avid one has a similar step involving the clamping of the tube at the caliper end though. tut tut!