Yeah you are right in that if one cylinder was at fault you should notice not much of a difference when you disconnect the
. If it's a problem with all four it could be down to and
fault it could also be down to something like the manifold pressure sensor or a couple of others but these faults should really show up when it's checked for faults.
Sorry the nearest specialist I could think of to you was Portsmouth which would be to far. You do have BB Tuning in Essex who do have a lot of diagnostic equipment and are quite clued up on there Renaults. If you are close it might be worth a phone call to see if they think they might be able to help.
BB Performance Tuning
Unit 4 Riverside
Essex CO6 2ER
Fax: 01787 220457