Couple of things I forgot to mention previously, firstly one ill effect of the mileage seems to be a gearbox that while nice to use when driving gently, gets uncooperative when down changing and pressing on. A few solutions have been suggested, including replacing the gear lever cables, clutch slave cylinder and the gearbox. I have a spare 'box from a much lower mileage car so may fit that in the near future - of course if we are going to do that we might as well also replace the clutch and slave, which sounds like more expense.
On the subject of expense, my rattle finding mission on Saturday morning identified that I have a blow from one of my flexi pipes, which will necessitate removal of the manifold, dropping the subframe, etc. again. What was especially annoying is that the starter motor heat shield was slightly loose and I spent over an hour with my arms stretched up behind the engine fannying around removing and refitting this for it to make no difference whatsoever.
The bad news is that a new manifold is £750+VAT. I could get a custom manifold made for around £600+VAT but the benefit would be limited as the standard 4-2-1 is already pretty efficient. With this in mind I have bought another manifold with damaged flexis which I will get replaced before fitting to my car.
At the same time as changing the manifold we will swap the ARB back to standard, and may think about swapping the 'box, etc.
On the subject of the manifold the faults are interesting, someone more knowledgeable than me might understand the reasoning, but the faults both the cracking at the flange and the failed flexi both involve the middle two pipes. According to the garage the flange cracking appeared to relate to the middle pipes twisting rather than the up/down or back/forth motion of worn engine mounts. I think the two flexis sit next to each other so one would have thought the leverage against the flange would be the same. Odd in any case and the commonality of the fault is concerning. I know of at least one owner with a car outside of the warranty period who had a strong goodwill contribution from Renault, but his car had done far fewer miles than mine, and my servicing has been done by an independent garage rather than a franchised dealer.