Sounds to me the small claims route is your best option, but first he has to give you the opportunity to rectify any faults / shoddy work etc.
I would suggest you write to him asking to detail in writing what he considers to be a problem, then work through each item one by one. It should be easy to prove if the rads are correctly sized or not, but the other stuff ie positioning of rads etc not so easy if you have nothing in writing.
If a magistrate sees you've made every effort to make ammends and be reasonable to the guy I'd say you stand a good chance of getting your money (plus costs).
And if he is seen to be unreasonable - it'll do him no favours at all.
Hope it all works out for you