all you need is a scalpel or stanley knife, electrical tape, magnifying glass and determination/patience..
so should this happen to anyone else i thought I'd let you all know that it wasn't the actual proximity sensor i had lost but a tiny 'cushion' with two holes the size of pin !$$&*! that went between proximity sensor and the front casing, i decided to try and make one using electrical tape which i cut to size with a scalpel and then very carefully made the holes with a fine needle (and magnifyer) equal spaces apart, all very fiddly and precise but it worked perfectly once my phone was reassembled....it's a lot easier than it sounds....