Noticed today that Halfords are now doing their own brand fully synthetic PD oil, spec mentions it's for VAG group PD diesels 505.10 spec.

5 Litre bottle costs

If its 505.01 then its good for PD's on fixed servicing, i.e Fabia.

Do halfords sell something suitable for Variable Serviced vehicles?

Not sure, I was only in a small branch when I noticed it.

I do know that Castrol look to have rebranded the SLX II longlife oil, something now called Edge from memory. There was guy there at the same time looking for the longlife oil and he was told they didn't stock it. Could be they do it in larger stores.

I used the Halfords stuff last time around, mainly because it's convenient.