Not sure what caused it - oil level was fine had checked before leaving the house. Wasn't driving it hard when it went - just in town at 30mph. Recon the oil pump could have gone but I have no idea.
Good news though! I have bought a 2.3 Mazda 6 engine and am currently swapping over ancillaries, sensors, clutch etc. The boy selling the engine had a Cosworth rep. inlet manifold and I already had the 60mm throttle body so hoping for a bit of poke once/if I get it up and running. Debating wither or not to swap the heads over - it would be a better job and give more power but I don't really have a clue what I'm doing!