Couple of things may be causing problem after replacing the group gasket. First it's possible you put the replacement gasket in up-side-down in which case it won't seal as well. Quite easy mistake to make actually. Second your basket may be a contributing culprit. Possible it's become slightly out of true or has a small ding in the seal lip. Also, did you clean the gasket channel thoroughly before putting in the new gasket.

Thanks for the advice all, I made double sure that I placed the seal in the correct way and whilst I had the machine apart I gave the machine a good clean at the same time. I hadn't thought of checking the basket though, so I will keep that in mind for next time I have an issue.

In all between replacing the seal and making making sure the the portafilter is in tighter I seem to have resolved the issue.

I have a suspicion that Mikes comments about the basket may have been the cause and the seal itself didn't need replacing (after pulling the seal I couldn't really visually ID any defect in the original seal), and I should have just double checked the basket if I had known!

Obviously changing the Silvia would be a sweet move... unfortunately finances say no and for the moment I'm very pleased with what I can get out of it so long as I am patient. Not always one of my strongest characteristics at 6am...

FWIW IMO changing your PF gasket was not a waste of time or money since you've had your Silvia about a year. Group gaskets are a regular maintenance replacement item. How often depends on usage: hours machine on and shots pulled. Regardless at least annually a good practice for a home machine. Much easier to replace before they become hard as a rock fully carbonized!