If you have a temp clamp, you can check the outlet pipe of the boiler while it is doing this. This would at least rule the boiler out. but tbh sounds like a mixer problem.
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