I guess you will need someone to take off the head and replace parts killed due to overheating, plus fix the leak... It's not looking good, I'm afraid to say - if the engine cut out due to no water/overheating then it's really not good.
Having said that, I once drove a Nissan Micra 1.2 litre home with no water in it and although it was stalling at the lights due to overheating, it did no long term damage and ran fine and lived many years once I replaced the radiator and filled up the water!!
Different category of engine though I'm afraid, the Twinspark is a more refined, hence 75bhp per litre instead of 50 in the Micra.
Did it die on the way home from buying it?? If so then the previous owner must have known about the leak.
To answer your actual question: Autolusso have a garage in Ferndown, Dorset... expect a head rebuild and £1000 bill... (but phone them for advice, they are friendly, it might be a lot less cost than this)