Update; GOOD NEWS albeit a little strange
I've studied my lenses against each other and against pics forwarded to me along with others that I've found online
In addition, I've gone over and over the lens mount of the body as well as the lens locking / release mechanism.
I've found absolutely nothing reassembling a fault.
So, perhaps stupidly, I decided to chance mounting a lens on the body ( it had to be done at some point ) so I chose the 150mm.
It mounted and removed smoothly without any problems evident.
I worked my way through the 105, 75 and 40mm lenses,....No issues with any of them.
The 50mm, the lens that originally jammed, I left while last, and indeed I was somewhat apprehensive about attaching it to the body.
It went on with no issues and incredibly, after cycling the shutter a few times, it detached just as easily.
I've mounted, dismounted all five lenses several times, cycling the shutter with each, and everything appears to be fine so what on earth caused the 50mm to "stick", I have no idea.
Similarly the "snapping" noise when it eventually released looks to have caused no obvious damage .
Much as I want to know why the jam occurred, I'm delighted and very relieved to have my ETRS back up and running