Thanks all for the helpful suggestions.
I'll have another go at slowly surfacing after work, but I can assure you that all flooding is repaired. I repaired it once and blew ballast, then got chopped to bits on the surface, suffered more flooding and sank like a stone back to 80m, where I again repaired flooding. No compressed air, both port and starboard electric engines destroyed. I tried the S-key repeatedly, and after repairing my stern all it did was raise my stern about 20 degrees. Quite a sorry sight, really, U-48 with her nose buried in the bottom at an uncanny angle.
Oh, and because the GWX positive bouyancy was leaving my conning tower a little exposed at periscope depth, I used the JSGM (?) thingy and a mini-mod to revert bouyancy to neutral
So I guess I am screwed......
Time to break out the schnapps, turn on the gramophone and prepare for the inevitable methinks
*Thanks for the pointers on getting through the Channel too - when I regain some confidence after this disastrous escapade I may attempt it again......maybe.....