Thanks again for your input. You confirmed my suspicions regarding the Toslink's capabilities, but unfortunately, due to the age of my receiver, I have few choices. As I stated, it doesn't even have a direct optical port (hence having to use an analogue to Toslink adaptor) let alone any HDMi sockets! It comes down to either just listening via the TV's internal speaker and having the benefit of Dolby Atmos, or foregoing that and outputting the sound through the receiver and my two stand-alone speakers.
Of course, the 'sensible' thing to do, would be to just go out and buy a new receiver, equipped with all the latest/current connections & ports and then....no problem! However, I can't really justify that and also, most of the ones I've looked at put out 100w (or more) per channel and my Mission speakers are only recommended for 25-50w pc and although compatible with the modest output of my ancient Sony receiver, might possibly either distort or fry with a unit pushing out more than twice the power, so I'd have to go to the additional expense of replacing my speakers if I upgraded the amp. You see how things escalate......? Additionally, I have no intention (with regards to my room size & layout) having any more than those 2 side/front speakers, so 'proper' surround sound isn't a requirement. I just want a bit more 'presence' to the sound when I'm watching a movie, than is output from just the TV speaker.
As I said, I'm hoping to upgrade to a 'basic' 4K UHD Blu-Ray player soon and quite possibly the LG UBK90 that I mentioned. I still have that analogue connectivity problem to work around, but it does have an optical out port and is equipped with 2 x HDMi ports which would stand me in good stead for the future IF I bit the bullet and bought a new AV receiver.
Fo the moment, with the TV, I've got great UHD vision and just have to compromise on the sound if I want to output it through the existing receiver. It's a trade off that I just have to accept I think. Just can't have it all at the moment!