Mike
Thank you for posting the letter.
On a first read and in my opinion, the letter as posted would be regarded as evidence that contradicts Jeremy's alleged claims that Nevill and June Bamber were actively discussing the possiblity of making arrangements for Sheila to have the twins fostered. Such discussions being the trigger for Sheila to turn violent and use the gun Jeremy had not secured safely after Jeremy had been outside to try and shoot some rabbits.
If Nevill had received such a letter from Colin such discussions, as alleged by Jeremy, with Sheila whilst involving June but not including Colin would only serve to alienate Colin from Nevill. In my opinion, it is also clear from the letter, as posted, Colin regards Nevill as a potential ally in resolving a matter that had been affecting his two young boys. As such, I would very much doubt Nevill would want to alienate Colin as Colin's letter appears to be very reasonable in terms of it being a request for help to a man who has earnt Colin's confidence.
In my opinion, the letter does not support and is inconsistent with the case of the defence.