Well, I am very grateful for all these comments, thank you for your time ... Yikes! It sounds very complex. Argh. Since my car is a peach, I think I might bite the bullet and register it....

Nigel, could you recommend the gestor you used...? Were you already a resident when you went through this process? Do you think this is obligatory....?
Also, Farway, sorry to be slow, but I would love to know what "an E.K.A.F mot" is ....?

Thanks again for the comments. I do have one perhaps useful tit-bit in return. In the UK, short term car insurance is available from a number of insurers, and I have found one firm that allows trips in UK registered cars to originate outside the UK, even if they are for one or two days. So that really suits weekenders like me. The company is insuredaily.co.uk.
Perhaps this is of use to some...?!