Thanks John, a lot to take in here... I must admit i did consider a time-delay rcd but these are at least £60 more than the standard, not a great deal if its necessary but I rather was thinking (or maybe hoping?) that the main problem is the old type of voltage -controlled rcd that is currently installed;
Does the earthing conductor connect through the device?
As you say it is 100mA then it is more likely an RCD as opposed to a Voltage Operated ELCB.
Not to get confused that RCDs were also called ELCBs in their early days!
My guess is that some sort of mains suppressor in a device is taking current to earth when there is a surge on the system.
As regards the new system not complying with 17th edition, Brian Scadden (the doyen of electrical text books) actually recommends installing a split load system for TT using 100mA overall and 30mA sockets protection in his book "Electric Wiring: Domestic", page 22, although it was written in 2005.
When it was written back in 2005 the 16th edn regs were the current version which permitted this setup.
as I understand it, you do not have to retrospectivley bring the whole installation to comply with the latest edition regs, just the bit one is installing - there are plenty of rewirable fuse boxes around which are not illegal.... however, I am op[en to correction on this, as you say one can advise the client.....
There is no requirement to bring everything up to todays standard, the regs basically say that before you carry out any addition or alteration that the earthing and bonding should be brought up to scratch and the existing system should be adequate for your additions/alterations. Have a read of regulation 131.8
As for rewireable fuses they are still a current standard (BS3036) and there are a lot about, although it is not the norm to install them in domestic premises nowadays.