Maris Pipers

Mine have started to flower already. Is this normal or due to the fact there's been hardly any rain for months?

Nothing stopping them flowering, but with maincrop it is not a sign that they are ready.

Flowering is used as a sign with early spuds indicate that there may be some of a useable size, not that they are mature.

Maincrop are generally grown to maturity and as is the case with all spuds, the sign of this is the foiliage dying down. There is a long way to go yet.