hi mate check the coolant level {not the header tank the enginge itself metal cap l/h rear side of engine as you look at it}as there is a fuel limit put on if the engine gets too warm there is a metal pipe on the bulkhead called a heater valve with three glow plugs in it these have a tendancy to leak badly masked by the fact the escaping coolant drips onto the exhaust down pipe it is worth checking the front to rear coolant pipes aswell these corrode badly and leak especially the centre and rear pipes around the spare wheel carrier
also pull the fuel filter off without losing too much fuel give it a good shake up blocking the inlet and outlet ports obviously and pour contents into a clear container old coke bottle etc and look for any metal deposits check with a magnet any thing lager than a deposit the size of a 5p or bigger could spell trouble for the high pressure fuel pump the injectors also have a common fault with the copper sealing washer burning out at the nozzle of the injector causing hot gasses to pass the injector and can cuse a bit of a misfire and a chuffing noise
hope this helps stu