Very many thanks for all the comments and suggestions regarding replacing our old non HD Sky box.
In the end I took what seemed to be the logical and easiest option, purchasing Freesat's own 4K TV Box, not least because it appears to accept a range of LNBs, and I have no idea what type we have.
Box arrived promptly, all plugged in and away we went. Impressed myself by connecting it to our Wi-fi, although Netflix would not work. Having read that new boxes can take a while to update, I tried again the following day, and sure enough, Netflix worked. Even used my phone with the on screen bar code!
One small niggle, which is probably me missing something. Living in Aberdeenshire, my default BBC1 channel is Scotland, which is OK, but there does not seem to be an alternative channel for BBC London. The only way, as far as I can see, is to delete the postcode after which BBC1 defaults to London, but without a channel to view BBC1 Scotland. There must be a better way to change BBC1 than keep inserting or removing a postcode?