I've discovered what it was after your help to eliminate the problem one by one.

It was the water feeder pipe not being fully wound in and tightened. Who would have thought!
Somehow even though it was screwed into a point, it clearly wasn't enough!
Now the leaking has stopped and I'm back with an awesome machine again.
Thanks for all your help and I Hope this helps someone else with the same problem!
Best wishes.

The water feeder pipe seems to actually have been a red herring.
No water seemed to leak with the shower screen and gasket off the group head after I wound the feeder pipe in more tightly.
The problem seems to now look like it it may be related to the position of the piston at the bottom of the lever travel allowing water in. So if I raise the lever just 1cm whilst the dripping is happening, the water stops leaking. So perhaps the end of the thread at the top of the piston bar is worn preventing me tightening the nut down far enough to keep the piston from being down too far in the group.
Nothing is cracked or damaged that's for sure! But what would having the screen and gasket in place make the dripping happen and why is the quantity always about 90ml?

ochapple wrote:The water feeder pipe seems to actually have been a red herring.

yes - it never made much sense as to how that possibly could have caused the symptom.

ochapple wrote:So if I raise the lever just 1cm whilst the dripping is happening, the water stops leaking.

That makes sense. This sort of drip is typically caused by a poor upper seal. Raising the lever would tend to cause the seal to flare and seal more tightly against the cylinder wall, which should stop that leak.

ochapple wrote:So perhaps the end of the thread at the top of the piston bar is worn preventing me tightening the nut down far enough to keep the piston from being down too far in the group.

That doesn't seem at all likely. Do make sure that your lever is not installed upside down. It should look like this:

Note that the forward pin should be lower than the centerline of the lever yoke. If upside down, the lever would not go fully down.

To adjust the topnuts, back them both off and tighten in the portafilter good and hard then remove it. Place your fingers against the shower screen while lowering the lever - you can feel the point where the piston is starting to push against the screen. adjust the lower nut down to where you no can longer feel the piston pushing on the screen when you pull down the lever, then tighten that lower nut an additional appx 1/4 turn beyond that point. Then put a wrench on the lower nut and hold it there while you tighten the upper nut down against it. locking the adjustment in place.

ochapple wrote:But what would having the screen and gasket in place make the dripping happen

I can see no reason for that - perhaps a coincidence - may be related to moving the lever a little and flaring the upper seal.

ochapple wrote:why is the quantity always about 90ml?

As the pressure in the boiler builds, that upper seal should flare and seal under the pressure. Maybe it takes about 90ml to hit that point.

Let the machine stay at pressure for a good while the next few times you use it. That may help get that upper seal softened and flared better against the cylinder.

Also, next time you remove the sleeve, check to see if there is a depressed ring or trough around the inside of the sleeve bore. Like this: Plastic sleeve ok? . The new replacement sleeves seem to have that. I doubt that would have anything to do with this, but who knows.

I bet on the piston seals....

I had the same issue on an old pavoni pro...I gess the piston seals are either too old OR too small...or wrongly fitted

Once you get some pressure it leaks...after the pressure increases it forces the V shape of the superior one to be pressed on the wall and thus sealing it. If you apply the proper grease it will solve it for some time.