Yea I'll do that.
The last time this happened to one of my sites it came back after a couple of weeks without me doing anything.
I don't think it was an algorithm change, a loss of weight to certain types of links etc because of the fact that it has fallen so far so fast without losing any "big" links.
Anyway it could be a million different things anyway, it just seems odd to me though
Thanks for your help and time Richard.