Osma Ring Seal Fitting Problem Imperial vs Metric?

[Ritchie52]

I am trying to replace leaking access plug on the end of a old Osma drain within the floor space in my house. The plug fits on the socket end of a 90 degree branch giving rodding access to a length of horizontal drainage pipe. I have purchased a 110mm Osma access plug and a similarly sized T-ring seal to do the job. The existing pipework including the branch is probably 40 years old and hence may well be 4" pipework. Whilst I can get the 110mm ring seal located into the recess in the branch socket (albiet with a bit if a struggle) there is just no way I can get the chamfered end of the plug to push into the socket past the seal. Even with silicone lubricant, there is huge resistance to pushing and if I do what I shouldn't do and employ a timber block and and a hammer, the seal gets pulled out of the recess.

Is this just a straightforward imperial vs metric problem and if so any advice on how to get round this without replacing all my pipework? Or am I doing something wrong with the ring seal. It is pushed fully into the recess and I have tried fitting it both ways ie tapered edge and blunt edge facing out - neither makes any difference.

I am really struggling so any quick advice would be very welcome!

Thanks