Yeah, just been looking at Google maps. I think it's a 25-30 min walk. Providing you get out by 21.40 and leg it (take the short cut down Adderley St) I think you should just make it.
Just checked on trainline again, 22.10 from New St to Euston is still £8. If you can travel up earlier in the day, there are London Midland trains from Euston for £6. So possible to travel from London to Brum and back for £14 in total. Bargainastic.
Or you can walk back to New Street - it's about 25-30 mins, so should be possible to catch that 22.10 train. Last time I checked, a single for that journey to Euston was just £8 - so no excuse for London-based fans not to go!