Differences in push-fit waste sizes


I'm just plumbing in a replacement shower tray as part of my en-suite re-fit.
The old push fit waste pipe has been removed from the push-fit connector on the soil stack and was a tight fit. The new 40mm pipe courtesy of Homebase slides in much easier and I suspect may not be good enough to be water tight. It appears on inspection the old pipe is a slightly larger diameter.
I've changed the seal in the soil stack for a new one of the same type (borrowed from a 40mm external grade compression joint) with no improvement.
Now there are two obvious solutions I DON'T really want to adopt because the soil pipe and shower waste pipe is going to end up behind a tiled stud wall until it gets under the shower tray.
1. Use a section of the old pipe into the soil stack and an in-line compression joint to the new pipe. The compression joint due to the position of the old shower would end up being well out of reach for any maintenance.
2. Just clag the joint with sealant. Again, it would need major open wall surgery to get to to it if I ever need to pull the pipe back out for any reason.
The way I'd prefer to approach this is:
1. A way of PERMANENTLY jointing the two types of pipe. I'm currently considering whether a 40mm solvent weld coupling will do the job (old pipe may not fit in + I think the pipes need to be ABS for it to work?) and no experience of the life span of the flexy couplings of the type screwfix do.
http://www.screwfix.com/search.do ;jsessionid=MA3KWUNFHMCNCCSTHZOSFFI?_dyncharset=UTF-8&fh_search383
OR
2. Fitting an oversized seal if such a thing exists (tried to improvise one yesterday from a compression joint seal but no luck so far......)
Thoughts?
TIA Midge.
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Just noticed - the old pipe is marked up as "Marley Extrusions 42 High Temp BS-5252". Sure enough the diameter is roughly 42 rather than 40mm.
Is it possible to get this stuff still as a long section of it reduced under the shower tray where I can actually get to it would also solve my problem.
Midge.

http://www.screwfix.com/search.do ;jsessionid=MA3KWUNFHMCNCCSTHZOSFFI?_dyncharset=UTF-8&fh_search383
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Ooops - should read BS5255

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http://www.marleyplumbinganddrainage.com
may help.
Adam
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Hi Adam - I e-mailed Marley tech support and got a very helpful and fast response from them this morning. My old pipe isn't manufactured any more (it's Polypropylene with a diameter of 43.05mm instead of the current 41.15mm) BUT solvent weld 40mm ABS pipe happens to be the same larger outside diameter.
Picked up a length of that this morning and the fit is perfect - so I can convert it with a compression joint in a more accessable place.
Thanks, Midge.

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That's what I like to see, a company that helps.
Nice one
Adam
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