Well I haven't solved the problem but now several days after my purple water experiment in the sink I can see a slight staining in the grouting between the paving slabs in the porch! Sorry I ever did that now 
Obviously it's not going into the pipe and is somehow just flowing out under the porch, those slabs and grout may have got damp looking other times but I would never really have noticed as in bad weather rain blows in there a bit and it's not unusual for them to be wet but this is a lingering darker shade in the grout lines. The only consolation is that patch is not the one where it sounds hollow underneath, it is directly outside the bathroom window and would be roughly where the pipe should be going to join up with manhole.
Anyway I will leave well enough alone for the time being, that's a job for summer weather as it will make a mess having to take up the paving slabs, might buy one of those little inspection camera thingies if they come into Lidl again to see if that shows me anything. Luckily the sink in there is not used very often so will limit it even more until another day!
Really annoying as I renovated that bathroom myself about 10 yrs ago, did the tiling, woodwork, shower enclosure, presses, everything bar the plumbing! Got the professionals in for that, ironic as the toilet is actually not fitted correctly either! Thanks for all the suggestions.
Obviously it's not going into the pipe and is somehow just flowing out under the porch, those slabs and grout may have got damp looking other times but I would never really have noticed as in bad weather rain blows in there a bit and it's not unusual for them to be wet but this is a lingering darker shade in the grout lines. The only consolation is that patch is not the one where it sounds hollow underneath, it is directly outside the bathroom window and would be roughly where the pipe should be going to join up with manhole.Anyway I will leave well enough alone for the time being, that's a job for summer weather as it will make a mess having to take up the paving slabs, might buy one of those little inspection camera thingies if they come into Lidl again to see if that shows me anything. Luckily the sink in there is not used very often so will limit it even more until another day!
Really annoying as I renovated that bathroom myself about 10 yrs ago, did the tiling, woodwork, shower enclosure, presses, everything bar the plumbing! Got the professionals in for that, ironic as the toilet is actually not fitted correctly either! Thanks for all the suggestions.