Great feedback thanks ... I'm sure the float valve is fine there is the slightest of drips but that is all.
Will spend the weekend trying to recreate the problem and repost if I get anywhere (happens first thing in the morning when usually on way to work hence why I'm a bit vague at the...
Having turned down the cold water pressure upstairs a tad more the problem has not happened this week... hot showers straight away and no overflow.
My worry is this (don't laugh as I really haven't a clue how such a device works :D) ... is it possible for the Thermostatic valve to allow the...
1) Boiler make / model (gas or oil)
Gas Open System
2) What is / is not happening
i) Hot water to shower/bath can take up to 2 to 3 minutes to appear
ii) Hot water input/overflow tank can overflow at various times sometimes coinciding with using bath/shower
3) Has ANYTHING else...