The shower tap in the picture is, I think, a Hudson Reed shower control, similar to this one
Can anyone tell me how I can dismantle this as it is leaking and presumably needs a new washer? Also, I think the lower tap has some kind of lock which controls the hottest temperature (there is a small 'button' at approx. 3 o'clock on the lower tap). What is this called and how can it be adjusted?