From posts made by Marc on Pentaxforums it's almost certainly the Pentax FA* 250-600mm f/5.6
with a K10D as well who needs a K-5

Sorry to read about your mountain bike accident Marc... Get Well Soon!!

Nearly all 300mm + lenses focus past infinity.... it's to allow
for thermal expansion and contraction.

Now gone up to £530

Minolta shouldn't need a lens for Pentax stuff. Minolta AF is 44.6mm
It does need a lens to use Minolta MD lenses they were 43.72mm.
The last one I saw was about £200 Ouch!!!!

They often use a Sigma prime lens that's available in all
mounts for testing.

Almost any lens will fit on a Canon because it's registration distance is small at 44mm.... Pentax is 45.46mm so no correction lens needed.

Nikon is 46.5mm so a correction lens is needed for infinity focus with Pentax lenses.

They have an Earthquake! Setting

Very nice pictures Andrew.... the 18-55mm always amazes me

Unless you have unlimited amounts of cash.... you will be able to
buy more Pentax lenses than Nikon lenses

I quite like it even though it suffers from a few problems listed
below.

The burnt out edges of the leaf are distracting. The picture does
show possibilities though, it might be worth a retake. The drop needs
to be sharper and clearer and (not really possible) bigger. A crop
would help, but the only way to really improve it is to throw in
some additional light (reflector or flash) to balance up the
shadows and highlights better. A diffuser held to tone down the
sunlight might also help. I'd keep the drop central.