Your system pressure is low so you are losing water from it.
If you are capable, undo the nuts from the external EV and the boiler PRV, and pull pipework gently apart.
One of them will be wet.
This will be the one that is passing and will need sorting.
Things are going to get a whole lot trickier as we near net0.
Low temp heating systems mean heat sources will have to be more closely matched to the heat loss of a property.
You won't just be able to sling in 30kw combi and whack it up!
Close both towel rail valves.
Crack one of the nuts from rail to valve.
Have a baking tray or hole less rubble bag around the rad tail.
Open air vent and control flow of water into recepticle by opening and closing the air vent.
When it's empty close air vent and remove rail.
You can hole saw one or both out to receive waste pipe of 50mm (or 40, or 30 with reducers) in your case... The new sink connection. The 'ears' can be sawn off flush if they are not used and are binding against something like a wall if the soil is very close to it.
It's best to run all waste...