Guys thanks for your assistance in getting this mower all back together. After a couple of hours work it is now running very nicely and most importantly not leaking any oil around the valve chest cover.
I ended up using all the new parts as you indicated AVB. The valve guides were very easy to replace onto the new metal plate.
I ended up lapping the valves before re-assembly however they were already in quite good condition. It was surprising to note the inside of the bore was unmarked which I found surprising for a mower that was 10 plus years old.
I have included some photos of the process which others might find useful.
[Linked Image]
[Linked Image]
[Linked Image]
[Linked Image]
[Linked Image]