Thanks guys I followed your suggestions.
- I set main switch to 'off', then RCD to 'on' - which had resistance this time and stayed on
- then put RCD back to 'off'
- then main switch back to 'on', RCD back to 'on' (now with resistance again), then switched MCBs back to 'on' one at a time (I thought this was correct procedure to avoid any potential overload)
- however, no power to any associated circuit was restored, and when pressing the test button on the RCD it did not trip the RCD
[*the above order now edited as I had made a mistake previously with writing the order, as mentioned in my next post below - my apologies*]
So would it now be correct to conclude that the RCD does need to be replaced, along with the tripped MCB before power can be restored?
If so is this Wylex RCD I've found ok as a replacement? It says 'DIN rail mounted', seems to have the same N&L configuration as the one that needs replacing, and from reviews the Wylex seems to be better quality than the BG one I found earlier:
http://www.builderdepot.co.uk/wylex-63amp-double-pole-30ma-rcd-wrs63-2.html