MPL

[Lone Ranger]

According to the CAA, there are at present 1487 valid MPL licenses.

I am trying to get an idea as to how many pilots fly microlights
on a PPL?

Anybody?

[skybound®]

I know of at least six mates that do, plus me - makes seven.

I think there is quite a high number of individuals doing this. One would probably need to ask CAA for a rating list as opposed to licence type, assuming that info is available.