Rover Diesel

I just couldn't resist it on Ebay. Rover 400 hatch with 76K on the clock. Picked it up at the week end. I was not surprised at the feeble performance as my experience is that if they've been standing and not done many miles a good few doses of diesel cleaner usually solves the problem. Ran fine until I put diesel cleaner in. Then started misfiring at 3000RPM if you close the throttle then runs perfectly. I thought it maybe weak so I pumped up the volume about half a turn now the misfire has moved to 3500 RPM. Any ideas what this could be? Water in the fuel?
Its the first time I've had any such problems with one of these amazing diesel engines.

sorry should have mentioned its a non intercooler engine.

im really losing it today. The engine management light comes on when this is happening but if I turn off the ignition and turn it back on the light goes off

Try changing the fuel filter as it could be fuel starvation (and turning the fuel up will just make this worse).