Thanks but actually I'm in St Ives now so will be returning to Paddington, boarding at St Erth. Given that is the first stop out of Penzance, I would imagine that we will be ahead of the main rush at Truro, St Austell, Par and Liskeard.I would guess try and board near the front, as there might be more seats there (using the assumption that some passengers are a bit lazy and will board the first available door at the terminus station).
There are some nice bus routes in Cornwall, over areas not served by the trains, I'm sure you will make the most of that day bus ticket.
If reservations are held then we're in car D.
I'm looking at the buses, and the L5 from St Ives to Helston looks interesting, and assuming the weather is kind to us, the 'Lands End Coaster' should be a good ride and very handy.