I do use a fixed set of IP addresses for the network
Saab, My normal network was shown in the network and sharing centre but it wouldn't connect to it until I disabled the public network, I could connect quite happily after that.
either way, the machine would have been self-allocating a public-range IP address rather than being allocated one by the router, so couldn't communicate
That is certainly what was apparently happening yes.
Anyway thank you gentlemen I shall just put it down to gremlins doing strange things and leave it at that.