I found myself, likewise with a PPL and no instrument qualifications, over gradually thickening clound a couple of weeks ago. Like you, I turned back ... and as I turned saw the top end of a radio mast sticking out from the cloud layer up ahead, not something I'd ever seen before. (I didn't return to base, I flew round the cloud layer and, after a bit of messing around, found my destination.)
Should I have turned back sooner? In my case the answer was yes, I should, by about two minutes. I'm not going to judge your answer for you, that's your job.
The "in sight of the ground" requirement seems to mean different things to different people. I've often heard that it means "50% cloud cover beneath you is OK". I've not often heard what you were told, that the occasional glimpse of the ground is OK. Having decided I didn't like it with 50% cloud beneath me I've decided that my limit is nearer 25%.
The question one should ask oneself surely has to be this: If the fan stops now, will I get down safely? If the answer is "no", then there's too much cloud beneath you.
[Oh, and one for the lawers: One can be legal, more than 1000' above a 50% cloud layer, but with lots of little clouds and lots of little gaps such that one couldn't maintain proper separation from the clouds were one to descend between them (all this well above 3000'). It's legal to carry on, not legal to descend through the gaps. Clever, eh?]