Called a lockshield valve (anti tamper) which is installed to reduce flow rate through the cylinder coil in order to eliminate short circulation through the coil when both heating and hot water are called for.
The tee would be either central heating return back to common return or simply a bypass although I would say it is the former.
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