hi guys so far things are going well im using as many parts from my ohp as possible, in the not too distant future im going to replace my fresnels due to being slightly smaller than the 15in lcd, ive ordered a 342mm fl triplet does this focal length have to match that of the fresnels im using?

Well, you'll get best results is you make the fresnel as close as possible to the lens that it's replacing, but there's some room for change, particularly if you can change where the lamp is mounted in comparison to them.