As the title really. I have some uni mould tooling gel left over can I use this to make a mould then Finnish the mould using polyester resin and fibreglass etc..

Yep. The Unimould tooling resin is polyester based anyway, so there's no problem there. I would recommend still using a vinyl ester skin cost for the first layer of fibreglass, as this will give a better substrate for the polyester resin to bond to than the vinyl ester gelcoat.