"...... Having taken on a small allotment (cost £8.50pa)...."

Stop thinking of the cost of the seed, they are still the cheapest part of
growing, think of the return per £1.00 on seed, should be £20.00 to £30.00
per packet, and if you swap plants with other allotment holders then could
be more.
Just think yourself lucky you pay such a LOW rent.