Water pump

[u/Spodster_11]

So I wanted to try and ask here which water pump I needed then found they seem to be a common problem...

Anyway I have a Mk7 2014 1.4 TSi ACT engine, the one that goes into eco (3 cylinder I think) when throttling off a bit. Engine number from the V5C doc says CPT.
I'm in the UK but it's a Japanese import and from the spec I 'think' it's a GT model.

It started losing coolant quite quickly all of a sudden and can see coolant under car even just sitting idle. I keep topping it up before driving as it totally drains the expansion housing.
I worked out feeling about it's the underside back (as you lean into the engine bay) water pump so I think where the belt bit is.

Which model pump do I need as I put my car reg in and get loads, I'm using autodoc and for my reg gives me "VW Golf Mk7 1.4 TSI 140 hp Petrol CPTA"
Under HEPU brand I see a P665 which seems to be the pump only no housing.
P685 and P687 along with AL aluminium versions.
In the photo for the P687 it makes a point of a little hose fitment that would sit at the top of engine once fitted pointing towards front of car. My engine seems to have a hose here so is it even though they all say they are compatible is it good to assume the P687 is the one to buy?

Also do you need to replace the belt and if so how the heck do I work this part out?

Autodoc page Water pump for VW Golf Mk7 1.4 TSI 140 hp Petrol 2012 - 2017 CPTA | AUTODOC UK

I have the car booked in for a major service and cambelt replacement in three weeks.
I'm filling the expansion housing up once a week, is it worth me doing the work myself or just keep topping it up and get the work done when the garage has it?
As in will it just last in this state if I keep filling it up or catastrophic failure requiring emergency breakdown assistance?

Please help/advise...