I would give it a good mulch with compost and a feed of fish blood and bone meal or something similar.

I think cutting back recently flowered shoots encourages new growth which will flower the following year

What time of year would this be Plug?

Immediately after flowering according to the beebs site but this would obviously prevent any fruit formation

Thats what I thought Plug. Just seen my neighbour in the village this is what he said/does:
Position in full sun or partial shade very tough and hardy. Quince does needs pruning, trim to shape after flowering, reducing last years growth by half, only cut back old wood if starting to be overgrown and leggy. He grows his as a wall shrub. He also give some, feed and mulches every spring though he did say it does well in poor soil.