I was not correct....
Just spend some time on various sites getting more information about the stem seal issue. It sounds like a big issue and there are many statements even from specialists.
Ok, let´s start.
P5 and P6 heads = no seals at all.
SD1 heads from 1976 = flat type (washer) rubber seal for the inlet valve. Exhaust valve no stem seal.
From 1976 to 1993 (for 3.5, 3.9, 4.2) the (bigger) black cap-style stem seal was a replacement solution for the flat one and requires no different valve guide. Only for inlet valve. Exhaust valve still without seal.
From 1994 the (smaller) cap-style stem seal came, together with a valve guide showing the recess. Now stem seals for all 16 valves.
MG RV8 engines = came with no stem seals, although available at this time.
Flat rubber seals = tend to split after short time, maybe result of poor OEM quality.
Black cap-style seals = tend to come off, no-one knows why.
Blue (latest) cap-style seals = you read a lot of seals coming off.
One well known guy from the forum here said, late series 4.6 replacement heads from Rover came with no stem seals as they might tend to come off.
Sticking valves seem to be a sole problem with the latest blue seals.
So, as my guides and valves are generally ok, I´m now a little bit confused what to do