Having been away a few days ... in hospital ... so I don't feel like doing much. I found the toilet overflowing slowly. I adjusted the screw to max but it still leaks slightly. So I am thinking that there is/was some debris trapped in the seat. It's not obvious how to access that area, in fact it may be best just to replace the filling unit. Are these easily available in somewhere like HomePro? ... tks

Yes.   Take a photo of your current unit or the model of your toilet to HomePro, ThaiWatsadu or Global House. They will stock genuine Cotto replacements. 

2 hours ago, JAS21 said:

Having been away a few days ... in hospital ... so I don't feel like doing much. I found the toilet overflowing slowly. I adjusted the screw to max but it still leaks slightly. So I am thinking that there is/was some debris trapped in the seat. It's not obvious how to access that area,

It looks easy enough to take apart, just rotate right about ⅛ of a turn after pushing down the locking catch. Though if it's old enough the plastic may have become fragile and break.