I can also recommend Tiger Helicopters at Shobdon - did both my PPL and CPL there (and no I don't work for them).
http://www.tigerhelicopters.co.uk/ if you want their contact details - they're one of the cheapest (if not the cheapest) as well.
Was in the same boat myself; relatively low hour PPL(A) when I converted. The reduced hours and additional exams have already been stated above. The real difference the A makes is that I think you're far more likely to be able to mass in minimum hours. Assuming you don't have a problem with the co-ordination and everything, you're presumably reasonably comfortable operating in an aerial environment, you'll spend more time concentrating on flying rather than enjoying the scenery out the window, and your nav and comms are probably OK.
Good luck!