Ok, so the results are in.....
Checked all three materials this morning, clear mastic, clear no nails adhesive and epoxy
resin. We also have the thin clear plastic card.
They had all set, but all were still a bit flexible, presumably cos I set them so thin.
They had all stuck to the cling film, so maybe next time I'll try greaseproof type paper.
But I could easily cut them into thin slivers, still attached to the cling film, into random shapes to simulate ice chunks, etc....see below.
The clingfilm actually seemed to give them a slight ripple effect was which quite nice.
The so called "invisible" no-nails adhesive had set white(ish) so is certainly not invisible!
Some parts of this were also a bit thicker than the mastic and resin, cos it washarder to spread thinly as its so "gloopy"!
However, I think this would be fine for simulating chunky snow or very thick pieces of ice, so I think I can still use it.
The Clear mastic was not that different to the resin IMHO, but was a bit more flexible.
It had set nicely and when broken up, and cut up seemed to form nice ice like shapes.
I deliberatley flattened it roughly, so there would be slightly thicker bits and slightly thinner nits, as ice is not always the same exact thickness, and you can see that some chunk have variable thickness which seems to work well.
They are almost completely clear, and have a nice rippled type effect, so these would be good for simulating ice IMHO.
Then we have the resin, and these imho look the best, only minot critiscism is that the resin tends to self flatten/self level, so it tends to be an even thickness, however it still looks pretty good IMHO. When broken/cut it seems to go into fairly random shapes.
Lastly, we have the thin plastic plasticard type sheet and IMHO this did not look that good, it just looked false and not realistic! I think because it is so even, so consistent and no weird shapes or ripples, which you do get with ice. Ice does have little variations, in colour, in shape and in thickness, so it just looks too clinical IYSWIM?

Heres the plastic sheet on a base and it just does not look right?
Here is the resin and mastic pieces on a base....

I actually used a mix of both the resin and the mastic pieces for these two little puddles, bear in mind this is an unfinished base, so once it has vegetation, and weathering and some snow and a white mist coat to simulate frost, you would hopefully get a much better effect.
Close ups of each puddle....
And one shot of the no-nails adhesive, which also works, just not quite as well IMHO.

So I think the winners are resin and mastic, but I could use the no nails too, but the thin plasticard did not look realistic at all IMHO.
Hope this also helps someone else out, cheers