We have used tectite metal a lot should be used with classic fittings.
Use tectite chamfer tool I've got set that fit in cordless.
Have used it with speedfit fittings on temporary jobs (site toilets ect) in truth never had a leak.
Used speedfit cap ends when pressure testing as well without...
We use eph stuff all the time very reliable but I still wouldn't fit a 3 port valve.
Your draining down and saving £30 against two 2 ports ain't worth doing .
Y plan = shorter lifespan and more difficult to trouble shoot.
Clear pictures of where open vent and cold feed connected to system would be useful.
Get a decent magnet and see if it 'sticks' to pipework in that area
Try pulling some sentinel X400 in system to clear sludge.
Not weird a common symptom of a tank feed system not working correctly.
Shaun has listed most common causes it needs sorting urgently while you still have a boiler and radiators that hold water.
As above 1 1/4 wall thickness depends on what quage normally medium or heavy,
It's probably a fair age that's a long thread jointing section by the late 60s unions were being used.
Unfortunately this seems to be the case with most water suppliers.
We work in 3 different water suppliers area's all are now on 'direct emergency call out to stop taps'
When I asked Yorkshire water to define dire emergency they said uncontrolled leak flowing out of house door!.
We install/service quite a few Grant's . To be fair at 7 years old it's not unknown for bottom baffle on a wall hung to need replacing.
I would get new baffles in a s a p
Only affects the logic range
A small number manufactured between August 22 and January 7th can be affect.
Advice from Ideal is to keep system pressure at 1.5 when cold.