The process is more of a legal one than a financial one, but the lender will need to be told nonetheless. As long as your mum can afford it then it shouldn't cause a problem.
Ask a solicitor to prepare a transfer for nil value. It's called a TR1 form and they will normally transfer it for about �50. Let the lender know that you will have no further interest and that your name should be removed, send a copy of the TR1, they might charge a small admin fee for the transfer but that should be the end of it.
If you already have a buy-to-let and your name is on another mortgage then technically you should consider let-to-buy but that won't solve your problem!
My idea is that you talk to a lender like Birmingham Midshires who don't mind you having more than one mortgage, you need 100% of the loan to value so check out some other lenders that specialise in let-to-buy as well. BM will give you high street rates considering your circumstances. Hope this helps.
I normally specialise in seperation and divorce but I thought I might be able to help here. Check out the site at www.redcubemortgages.co.uk it has some helpful tips.
Any more info required e-mail or stay on this thread and I'll keep checking.