Ermm.....
If it is hesitatiing at low RPM and has low power etc, could be valves(but this would cause other probs like bad starting), timing belt slippped(unlikey but if tensioner bearing failed it's possible), but sounds to me like it might be ECU related.
I am wondering if possibly the MAP (Manifold Pressure Sensor) is stuffed, and therfore load is not being correctly detected hence timing and fueling stuffed.
As mentioned in the previous posts, there is probably a whole bunch of stuff it could be.
The rover 800 haynes manual has some info on the T series engine, including Fault finding (which to be honest is very generic and a bit useless in haynes manuals). There is also a haynes manual for Engine managment systems which covers MEMS (again not exactly complete, refers mostly to implementation on K series).
I would suggest that the best bet is to search for an independant (or main dealer if you don't mind the price) who can check ECU fault codes from MEMS. This should either detect or eliminate any serious Engine Management prob.
If ECU is ok, then could be something as simple as inlet manifold gasket air leak (which screws up fueling if big enough).
I can suggest a few first pass test / Inspections:
Check all the vacum hoses for splits or disconections. Particularly the one gong to the MEMS ECU (this feeds the MAP sensor, infact you can do a really ruff test on the sensor by diconnecting the hose from the inlet manifold and then gently sucking on it, hardly scientific but if it is working to some degree you will hear the engine note change)
Check for problems on the HT leads could be one of the leads, or the king lead is discharging to the block. (also check dissy and rotor for signs of arching / wear)
listen for a whistle / whine of air being sucked into holes in inlet manifold gasket (not easy to detect, sometimes spraying something like WD40 around the gasket helps in detection because in momentarily seals the hole and engine note changes).
Make sure all sensors are connected, particularly the temp sensors and throttle pot.
Check fuses make sure it isn't something dumb like blown fuel pump fuse.
Check for mayonaise in oil or oil in water.
If there are no obvious split hoses or broken wires then just check the timing belt, like I said I dounbt this is the problem but better safe than sorry. This can be done fairly simply.
I can tell you how to do it, but you would be better off buying the haynes book, cos it has pictures. Basically you just need to lock the fly wheel (there is a small hole to do this just below and to the right of the turbo, I usually usa a 6mm allan key to lock it).
Once the fly wheel is locked take off the plastic cam belt cover and check the cams. You are looking for alignment of the Inlet / Outlet marks stamped into the cam sprockets.
