Toilet not flushing properly . Advice please

[woodman-46]

Hello,
We have a toilet in upstairs bathroom but it constantly gets blocked. I think that the cistern in photo is about 6 litres ? The waste pipe on outside wall is not at a great angle in fact hardly any angle at all.
As seen in second photo the waste pipe is above the front door and not by much and so we can't increase angle of it .
( The toilet waste goes into left hand side of this pipe in photo)
Also we live in a listed building and have been told that we can't have a separate waste pipe going vertically down from the toilet outlet to the ground.
Should we get a toilet with a bigger cistern to create more of a rush of water ?
Any advice would be really appreciated.
Thanks

 

[Diver Fred]

The outside looks to be plastic and unsupported - it probably flexes when the toilet is flushed which stops the waste and water from flowing properly. I'd make sure the pipe is supported at no more than 750mm intervals (500mm would be better).

 

[CBW]

Is it just the toilet that blocks? Does it actually block, or does the water rise in the pan?

 

[Schrayder]

4 inch plastic is fairly stiff. Fall looks ok.
Can you video the pan as you flush?

 

[Hugh Jaleak]

External soil pipe looks fine, it wont sag when flushed, it's too rigid for that. I'd be interested to see the outlet of the pan and how it connects to the soil pipe, my betting is the soil pipe was fitted to suit the original pan, thats now been replaced and it's on a slight uphill leaving the pan through the wall, which is probably where you're issue lies.

The 6 Litre flushes dont help matters either.