looking at that mate the easiest and quickest way would be the saniflow like lee has suggested, take out that bottom bit of soil pipe, it looks like an access plug or something similar and put in a branch or strap on boss that'll take the 40mm from the saniflow, you could do it in 4" along that back wall but its got a good chance to make fitting your new units a mare, and as above don't forget a whb and run the saniflow waste on its own, don't branch anything into it lol