it will be fine after pruning. do you know which variety it is? if not when does it flower?

Spring flowering ones are usually pruned after flowering.
Summer and autumn ones are usually pruned to about 12" from the soil in mid spring before the shoots break.

This is the RHS link
https://www.rhs.org.uk/advice/profile?pid=109

As the old saying goes-Clematis like their feet in the shade and their head I the sun. Your problem is that you have given your Clematis feet which are too big or long as it were. What you did was the following...you fixed the trellis against (judging by the shadows) the north side of a small stone fence or wall which gets little sun. Then you placed the plant at its base and that’s good but what was not good was that the fence or wall is too high. I don’t know how it did it but this plant finally managed to make it all the way up to the top of the wall where the sun was to give it enough energy for sustained leaf growth. Sad to say that there will probably never be enough of mother nature’s little solar panels to provide enough energy for flowering. Simply said you planted this Clematis in a bad place.

Thank you very much for the comments / advice. I’ll use the link regarding pruning. It hasn’t flowered at all and from what I’m reading might not ever. Unfortunately, I have very limited space and this wall,( indeed north facing) is the only location. I’ll nurse it along and see what happens. Entirely not suitable for its location being too shaded, but it was a gift, so worth A try.
Regards

Maybe you could grow another plant in front of it that doesn't mind the shade and hides the stems.

Just an update to the Clematis question.
I noticed that the lower leaves seemed to be much more susceptible to damage and wilt so I checked over a few days and discovered that I had a serious slug/snail/long black shiny creepy thing problem. Tried all the mechanical tricks like putting barriers round the stem at the base but then realised That of course they can climb the wall or the frame at will, so I had to resort to slug pellets which slowed them down at bit.
The growth at the top is very vigorous suggesting the plant is basically healthy, but I have this creeping leaf wilt from again bottom up where the leaves get brown marks then turn yellow and die. If as you have suggested this is likely to be location or other planting fault, then I’ll just leave it to take its chances. If however it is related to something like poor soil condition which I could influence, your advice would be greatly appreciated.
Regards

I find that Clematis are very forgiving when it comes to types of soil so long as it retains moisture. Mine seem to love growing in a soil that is predominantly clay. I fertilize mine with a liquid fertilizer once a year at the beginning of summer.