does anyone know exactly what is involved in removing these as apparently it is a known thing for these to corrode, rot and cause all sorts of problems. do you just cut the plugs off and then solder one set of wires to the other?

 

You cut them apart to extract as much cable as possible and replace them with good quality WP (weatherpack) connectors.

 

You can cut themof and replace them, you can get original connectors, just a case of knowing where, I use AMP CONNECTORS for example on my cossie to replace worn out loose pin original 18 year old ford ones, these are not expensive either.
DO NOT UNDER ANY CIRCUMSTANCES USE SOLDER, NOT EVEN SILVER SOLDER ON THESE, they MUST BE CRIMPED , and with the correct CRIMPER, NOT A BOOT MARKET £1 SPECIAL.
This is of course assuming you want reliability, I was recently looking at a £1200 wiring loom that had been bought from TVR before their demise, due to TVR having running problems.
The NEW LOOM HAD TWO BREAKS IN IT and disconnected wires which had simply pulled out of the crimp, it even had earth wires not connected, work done by TVR.
Needless to say this was corrected with a NEW LOOM HANDMADE ON THE SPOT, and car now runs perfect.
tabetha