I think my sump gasket is leaking slightly.
So does mine after I didn't replace the gasket but expected it to seal anothere 35 years.
I’ve found a post but not clear if the cover
will come off with the original 4 pipe system in place?
Depends, if the pipes are fitted correctly, it will, but I suppose there's enough room to get it down anyway.
Also once it’s off will the strainer just fall out and
Therefore need holding up when reassembled?
Last time I was there, the strainer fell out, but it was not much of a problem to position it when I mounted the pan again. Can't remember it fell out at a previous time time some 20 years before.
Looks like the strainer has an o ring worth replacing
while at it?
I have no knowledge of such an O-ring.
Tip: when reassembling use a good hexagon socket and if possible a T-handle to tighten up the ten bolts and reduce the risk of damaging the heads. Tighten them in a X pattern.