I am getting no hot water to my shower or bath but I get hot water in my sinks. I have a combi boiler which got installed about a year ago with all new pipe work. The bathroom got installed about 17 months ago with a mixer shower. I have had no problems until recently. I have seen comments on other sites saying the valve or washers might need replacing in the shower but why would this affect the bath? And surely with the bathroom not being that old I would not need to replace the valves or washer?

Your tub/shower mixing valve may have debris in the hot inlet. You need to close your supply lines and open the valve. Remove the handle and escutcheon plate. Remove the cartrdige from the valve body and clean it. Before reinstalling grease the inside of the valve body with plumbers grease.