MK1 .22 was my first PCP, superb rifle I stupidly sold it to buy a .177 (although the S510 I got was very nice) I loved the in-line "magazine" that you could top up as you went along, I'm rather surprised no one has used a similar design (a mate had a break-barrel with a similar tube mag that sat on the chamber back in the 70's).
Of course the mag is also the main drawback as it limits pellet design/choice.