Many thanks for the useful advice. My practice session proved most
successful and I was pleased with the results. However, things have
gone downhill since then. I had another go last night and something
has gone very wrong. As I spread the plaster across the board I am
getting scratch marks. It appears that the plaster has gone gritty and
the small pieces of grit are leaving scratch marks as I move the float
across the board.

I've concluded this could be because:

1) Plaster not mixed properly. I tried adding more water but this did
not seem to help. I threw out one mix and started again. Same problem.
Bucket was fully washed out between mixes.

2) Plaster has gone off. The bag of plaster was left open inside the
house without being covered up. Would it go off in a day ? The house
is not damp.

I am determined to master this!