Problems with tap's cold water cartridge

[Phil157]

Hi, my 2-handle kitchen mixer tap started dripping cold water. I've got it apart and am waiting for a new cold water, ceramic disc, 1/4 turn, cartridge to be delivered. Meanwhile I can't completely turn off the isolating (or service) valve, whose screwdriver slot is now knackered. So, with tap turned off there is a fast drip/stream which is worrying, but is why I've shelled out £18 (it's got a long spigot, total length is 57 mm) for a new cartridge.
More worryingly, with the tap's handle and cover removed I found that the flow can be completely stopped by pressing upwards on the end of the exposed cartridge spigot (where the splines are). What could this indicate? And do you use any silicon grease (on the O-rings, maybe) when sliding a new cartridge into position?

If I also decide to replace the isolating valve - 15 mm push-fit, plastic, with just a screwdriver slot, no lever - is there a good DiY method to squeeze the plastic end sleeves into the body in order to slide the valve off the pipe, in the tight confines of an under-sink kitchen cupboard? [photo attached]
I can just get a 5/8" open ended spanner over the pipes and was thinking Spanish windlass sort of set up to keep that spanner pushing inwards.
Thanks in advance, Phil.

 

[Knappers]

They wear and that sometimes causes a leak.
Can occasionally be packed out in a pinch by adding something to the rubber seal in the base of the cartridge (like a few wraps of ptfe) but not generally worth bothering.
The valves should come off OK. You could disconnect pushfit fittings underneath for better leverage if you struggle.
I'd consider taking off the trap for better access too.

 

[Phil157]

Thanks Knappers.
Yes, good point - definitely remove the lower plastic pipes, inc.the w/mc take off, first.