This gate valve position....

Excuse my wonderful drawing.

Someone put a gate valve here. If I've used the wrong symbol for a valve forgive me you'll have to imagine the right one.

There isn't one anywhere on the cold feed from the CWSC.

Since - if closed this isolates the water in the cylinder from the open vent - my guess is it is against regs as hot water has nowhere to expand.

However my question is....As long as the water is cold/hot water turned off can I use it to isolate the hot just to change a tap?

Not seen one in this position before and don't won't to overlook something. Can't think of a reason why that wouldn't be ok but want to check

Also, bonus question. If the hot water was off but still had say a bit more cooling to do before it reached where it was going to stay - and this valve was closed - would this cause any issues?

If it's dodgy to use I'll just drain the CWSC, fit one on the cold feed.

You are correct, it should definitely not be there.
It wants moving mate.

Ok to use to change a tap if water in cylinder is cold/hot been off for hours and book her in for re-doing properly at later date?

Take the handle off after you use it

And pray the spindle doesn’t snap.

But yes fine to do as you intend- as long as you verify it is open when you finish.

If it snaps you’ll have to remove it without delay.

Thanks all. Did the tap and took off the handle and advised the customer.

Out of curiosity though any thoughts about my other question in the OP?

"If the hot water was off but still had a bit more cooling to do before it reached where it was going to stay - and this valve was closed - would this cause any issues?"

If hot water expands then surely it's going to contract when it cools and if the vent is blocked could this in a worst case scenario bring about the same risk of cylinder implosion you can get from opening a tap if the vent is submerged?

Expansion/contraction can still be taken up by the cold feed unless you closed that valve?

Ah yeah...no there was no valve on cold feed.

if its a cheap gate valve it probably wouldn't shut off anyway