Neff Double Oven - cooker fuse trips out after 5 minutes?

I had a new fuse board installed in June 2007, as part of the installation of a new kitchen.
I had initial problems with the board intermittently tripping out, but this was rectified after being identified as a manufacturer problem.
In the last month (March 08) both ovens of my Neff double oven trip their fuse.
The rest of the board seems to be OK.
This does not happen straight away, but after about 5 minutes of operation, which seems a bit odd.
I first got the ovens checked out by an engineer.
There is nothing wrong with the ovens.
The engineer did find that the oven had been incorrectly re-installed into its new housing by the kitchen fitter, so that it was left sitting on the cable, and that the cable was now damaged.
This was rectified today, but, unfortunately, has not resolved the problem.
(I suppose that it was too much to hope that I’d get the oven repaired on my Birthday!)
So, I’ve covered the ovens themselves and now the cable.
Despite many attempts, I cannot get hold of the electrician who installed the fuse board.
He was only here as he was part of the kitchen installation company, but seems to have disappeared, since, and ‘no longer works for them’.
(Things that make you go hmmm...)
The next immediate steps seem to be to try a different circuit breaker, which I’ll try to do tonight, and, if all else fails (so to speak) to contact the appropriate supervisory body, (ROBIN) thereafter.
Or should I just find a decent local electrician to investigate it, and be done with it?
It would be good to be able to pinpoint the problem and the liability/warranty issues before doing so, but we are missing our roast dinners!
Anyone here got any useful advice as to how to proceed?
Many thanks.
Linda T
ps. I love this site; it has been so helpful and informative.