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When I apply the front brake I get a knock through the right hand side of the handle bar and you can see the fork moving back and forth slightly at the lock out side dust cover seal when the fork is in the locked out position. If I turn the lock out off and press the forks in grease comes out the dust cover seal on the lock out side. If I change the dust cover seal will this solve the knock or is there other seals in the fork that will need replacing.
Is there play between the upper legs and the lowers? Apply the front brake and rock the bike back and forth and you may see, or more likely feel play. If so the bushings (a sleeve inside the fork legs that the uppers slide on) will be worn and need replacing, generally a bike shop job
Very common issue with the xct range. They have a plastic insert that acts as a bushing and it is replaceable, however first you need to find a set online, then have it fitted by your lbs, probably with other work too as you mention seals leaking, then you are back to having a 60 quid fork that works for a short period of time until they wear out again.
Personally, I would save the cash and look for a good condition secondhand fork with a better build quality. You can get a set of Rockshox Recons or even Rebas for around the hundred quid mark.
I'd expect those forks to have various knocks and clunks even when they are working properly.
The fact that you can see grease coming out is actually encouraging, as it suggests there is some grease in there in the first place.
As above, don't pay to get them serviced, put that money aside and look out for a decent secondhand pair of recons/rebas/sektors.
As an example from the first page of the classifieds of what you might get for the money, obviously I don't know what your frame/wheels needs.
https://singletrackmag.com/forum/topic/90-rock-shox-sektor-gold-solo-air/
Or if sticking with suntour upgrade to a new model XCR air fork or Raidon or Epixon.
The cheaper suntour forks are cheap for a reason, as are the cheap rockshox forks.. The xct is suntours cheapest mtb fork and not really worth servicing.
XCT slider sleeves are £6.95, easy to install
Have you got a link to where I can buy them for £6.95.
LBS can order them from Greyville, normally some for sale on online; ebay, jejames etc.