So I set about checking the LHM tank --> HP pump pipe for air leaks, it looked OK. So if it's not that it could be crappy LHM with dirty LHM filters. I set about removing the tank and filters, words couldn't describe the crap in the LHM filters - so I took a photo!

The LHM in the tank was a disgusting murky brown colour, and with the filter in that state it's no wonder the PAS was heavy

The return filter was much cleaner but still filthy. After cleaning up the filters and putting some hydroflush in the PAS is much much better!