That would seem to eliminate tank fed shower, but adds a boiler problem.
Are you sure you adjusted the dhw setting on the boiler and not the ch setting?
with the dhw setting on it highest the shower temp changes quite considerably from its coldest point (where the water is warm but not too hot) to it hottest point (where the water is too hot to put my hand under for more than a couple of seconds).
I guess this means that there is some cold water coming through but not enough?
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