Hi Ben,
2 things spring to mind (both unlikely)
If you've not played with anything since it last ran
1. Timing belt. (broken or jumped)
2. Fuel contamination (I had issues with my k series running very badly, since it was after an upgrade though DVA/Northampton Motorsport had made an error, turned out to be Tesco's fuel. Maybe fuel is the issue here, also during upgrade found the fuel line from the bulk head to the throttle body(s) had perished (on the underside, out of view) could be sucking air in. If its the fuel line Earls at Silverstone will make one up using the old unions.