Defender TD5 cuts out at high revs - can't do MOT emission test

About a year ago my fuel tank was incorrectly filled with petrol and the van had to be recovered. the system was drained by a Landy mechanic and I went back on the road. Almost immediately the engine lost power and cut out with the engine warning light lit. To cut a long story short this continued to happen quite frequently, sometimes when accelerating but not always. My mechanic couldn't explain it and suggested I change the engine to a TDi! This summer the problem became so bad the van could barely be driven without cutting out, but would always restart after about a minute. I started to look on the forums and the issue with oil reaching the ECU came up and so I replaced the injector harness and thoroughly cleaned the contact points. I also opened the ECU and there was no sign of any oil ingress. After this it was like driving a different vehicle. As expected I continued to clean oil out of the eco contacts until all of the oil in the loom had worked its way out. The ECU connections are now clean and free of oil. However although things were vastly improved the van did cut out on a few other occasions, mostly but not always when accelerating. I took it in for its MOT the other day and the technicians were unable to complete the new emissions test because the engine cuts out before they can fully floor the throttle. I took it home and as suggested in some of the forums freed up the turbo waste gate, disconnected the MAP sensor and checked the hoses. Although it seems to be driving better than ever, very smooth with increased power, when I try to rev it up it still cuts out. Next I'm going to clean the crank position sensor but I'm not a particularly mechanically minded person and just following youtube videos and the Haynes manual. I would love to figure this out before taking it into a garage but I haven't been able to find any forum threads where people are experiencing this particular issue. I'm guessing its either a fuel system or turbo related issue but can anyone point me in the right direction?

You'll get better help here https://www.landyzone.co.uk/forum/defender-90-110-130.7/ and here https://www.landyzone.co.uk/forum/td5.50/