For about a week now I have noticed a tyre rubbing noise coming from the front tyre as I spin the wheel – but the tyre is not touching anything.
So I started thinking that I had somehow left a valve cap or similar in there. I took the tyre off tonight and found all my wheel milk had formed one big lump and has been rolling around in there and not earning its keep.
There was none on the tyre inner and very little around the tyre edges.
Is this a common occurance? Has this been brought on by CO2 usage?

Not sure about the CO2 usage but it must be a consideration. Yeah some times the latex solution congeels into a lump after frost or old age. Most of the new solutions are suposed to have frost protection additives etc. Not sure in reality how much it helps. FYI I took a pair of tublees tyres off my bike that had solutiion in them to replace for some faster rolling ‘summer tyres’. Left the removed tyres leaning against the garage wall for about a week. the solution ran to the bottom and pooled in the tyre. When I finally got round to wiping it out, and it was one large lump of gooey rubber!