@DarrenDevThanks for getting that working again.
Taken on board your advice about a newer device being more capable and faster than the first gen and ordered myself a third gen as a belated Christmas present to myself. The clincher was the Discovery+ service being less likely to be made retro-compatible which would impinge on my superbike watching this year - until and when there is a Discovery+ app released for the BT Youview box? (bearing in mind the superbike coverage sometimes is only available as an online stream and gets kicked off the main channels by tennis and cycling etc).
The old one may still have a use with my bedroom TV so not a total waste.
Many thanks Darren. Just tried it & it's back, but slow. It may well be weeks ago when I first experienced the issue but my wife's iPhone worked it. In her absence I got a Firestick to access BTS but found this post & replied beforehand. Chromecast will be retired.
For the benefit of others thinking of upgrading their Chromecast dongle I have just learnt that the one to go for is called Chromecast with Google TV as that has all the common service apps like iPlayer, Youtube and yes BT Sport built into the dongle itself which makes them more controllable and responsive. Can still cast from more niche apps from your phone and mirror-cast.
Comes in either HD or 4K versions depending on the current / planned resolution capability of your TV though if you are thinking of getting a new TV then built in Google TV is a consideration.
(I got lucky as the 3rd Gen dongle I ordered was with a recalcitrant Amazon supplier and got the order cancelled - now ordered one of the above)