Just to keep you all informed after excellent advice from smithy I took a slow drive over to Zworld in Tamworth today. http://www.zedworld.co.uk/.(And a very quick drive back) After a complete diagnostics check, it turns out knocking noise is due to a cam bearing, and the flat spot was a combination of two faulty lamber sensors a massive boost leak and one Throttle Body opening before the other. So one oil and filter change later lamber sensors disconnected which apparently was the best thing to do until they are repaired. Boost leak found and cured. I now have a 300ZX Twin Turbo that puts the biggest grin on my face-Since the day I lost my virginity ;) So many thanks to Geoff and his son for their work today and all for the poultry sum of £121 (Well worth the money)
Next stop back to Geoff at Zworld for new Cam Bearing, New Lamber Sensor and a possible chip upgrade. Thanks to everyone for all your help and advice.
Dave