MRC are held in probably the best regard for the 4.2 and can work wonders with it. They're only in Banbury so less than 90 minutes away unless you're in the east end.
Went to a Dyno and asked about tuning a V10 R8 once, as mentioned above there is little to gain from Re-mapping a N/A engine unless you have played around with internals. I would say save your money just put a good exhaust system on it to make it sound faster and better .
OK, the remap in isolation will not yield large gains but with some fairly minor, accompanying breathing mods you can unleash a very worthwhile improvement.
The V10 does actually gain a decent amount from a map as it was left slightly tame from the factory.
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