Many thanks for the quick responses.
The battery is new, but I also tried it with my Safrane battery which is new and fully charged. Nothing unless I use the jumper cable to the engine block.
What puzzles me is I can't find any earth connection between the chassis and the engine block. When I checked my wife's megane coupe there is a plastic coated braided wire earthing lead coming off the chassis to the block, so I kinda expected to find the same on the Clio, but no sign of one anywhere, or of there ever having been one.
I put a continuity meter on it and it shows a connection between the chassis and block, but clearly there is a lot of resistance there, which explains why the jumper cable makes the difference. I've replaced the earth lead from the battery and cleaned the chassis connection down to shiny metal, as well as the battery terminal.
I don't have the first idea how the
/Immobiliser on these things work, but I wondered if the earth was somehow controlled by the immobiliser, so if immobilised the earth connection is disabled, and if not by the immobiliser, by the
. Having said that, it would be a pretty lousy system which could be so easily bypassed by something as simple as jumper cable.
My other and last thought on it, is that when I started the car with the jumper cable earthing the engine block the throttle misbehaved, and I wondered if that indicated a fault with the
, or that by using the jumper I had somehow isolated the
. (Oh for the days of a normally aspirated engine, with an
and NO computer).
Obviously, I have been disconnecting and reconnecting the battery more than a few times whilst working on the car, so possibly that has effected it?
Anyway, thanks again for the suggestions and help.