Coincidence - look @ the drain run for a blockage - probably fat as you`re in London - the whole system has tons of it - from individual bedsits to the Embankment sewers uneducated public
I tought of blockage too.
But the toilet wich is on the back of this one is working fine (the back flow has happend from this one).
All in total it looks like the water pressure is not enough to open the lid on the abr until the level reaches the rim of the pan.
and than it is only enough to keep it open a little so the water has to squeeze through.
I can only think that the spring is too hard/strong . which makes it usefull when installed on a vertical connector.
Of course my one is horizontal (back outlet).
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local