Thanks for this Big K, I was thinking about getting the mains waveform checked.
There are some ‘mains conditioning’ devices on the market. I already tried one from Studiospares
https://www.studiospares.com/ProductDetail.aspx?p=517330
which has no noticeable benefit.
Furman make a couple of different units, there isn’t much detail of what they do, but it seems more focused on spikes than on ac ripple.
We do have stepdown transformer close by (about 15 metres from the end of the building) so I’m going to contact the grid distributor for information.