Leaking 3 port blending valve to UFH

[zooankski]

Noticed a slow drop in boiler pressure recently and think I have found the culprit! Looks like my blend valve is leaking.
I have sourced another valve which is 3/4". The original may possibly be 1" ? but they are not made anymore apparently according to Uponor, only 3/4 and 1 1/4.
I don't see this as an issue as it has 22mm feeds.
My question is, if I isolated the UFH circuit which haas 2 x 1" ball valves and turn off the power, will the zone valve hold the pressure in the rest of the house circuit (standard rads upstairs and flow in from the bottom) or am I looking at completely draining down?

I

 

[muggles]

Your photos are too zoomed in to make any real sense of them. Can you stand back a bit?

 

[zooankski]

Picture from further back. The UFH zone valve is below the leaking 3 port valve and actuator. The two isolating valves for the UFH can be seen on the right with red taps.
The question is if I remove power from the system so the zone valve stays shut and isolate the UFH, would I need to drain down the rest of the house?
It is a large chalet bungalow and the last time we had an engineer in to change out the hot water cylinder, he had a game getting the air out. In my previous properties I have never had issues doing this sort of thing but am slightly hesitant this time.

 

[ianmcd]

Below the lower red handled valve, there is another 22mm pipe connected, this will still be live even if you close everything else

 

[zooankski]

Good spot! Can’t believe I failed to notice the return! Guess that answers it then - cheers