Nuisance Trip

Hello to you all
I am a new member so please gentle with me.
My background includes an apprenticeship taken many years ago and following many opportunities I became a Service and Installation manager.
To save on wear and tear of this ageing frame I have decided to return to the tools (domestic only).
I have recently passed my 17th edition course and have the next two plus weeks booked on a training course reminding me of all the things I have forgotten and taking Part P.
I was asked to help a friend in need by fitting an extractor fan in his bathroom of a flat he is about to rent out. There was already a fan heater (fitted incorrectly) that he wanted removing so it seemed very straightforward.
On disconnecting the fan and trying to pull the cable back through a hole an RCCB tripped. This reset and I had no further issues. The RCCB is use to protect the whole installation it seems as the cu only has mcbs in the board.
I ran some additional cabling for the extractor and connected into the old heater circuit that looks as though it is a spur. There is also a DP cord switch and a separate RCD fused spur.
On energising the install the fan worked but after running for a while when I switched it off it tripped again but the mcb did not.
Is this simply a N and E fault on the original cable or am missing something?
Thanks in anticipation.
jamer8