Hi Mum2000, yes, i have seen this before and it's really annoying as you know. In my experience, an electrician won't be able to trace the fault unless they are really experienced as it tends to be due to electromagnetic Interference between the mains cables and the phone lines. There are the obvious things to look at like fitting micro-filters on all your phone lines, but, if the BT Engineer came and said there is a wider problem, then there probably is.
The best thing may be to talk to a home Networking specialist; i know one that may be able to help so drop me a line
mike@ensoul.co.uk or call me on 020 3637 0700 and i will see if i can help, all the best, Mike Lander, Director,
www.ensoul.co.uk