First pass seasoning done! Not as much smoke as I feared and there is a lovely black sheen on the dutch oven now.
I went with sunflower oil in the end - what I thought was rapeseed oil was actually grapeseed oil, which has a much lower smoke point than sunflower (216C vs 232C).
I reckon another two seasoning cycles and we should be good to go: the Ronnie Sunshine ovens do seem to be fairly roughly finished (although I know some people argue that's useful for giving the seasoning something to stick to). Either way it's noticeably smoothed out after the first seasoning but a couple more (plus regular meals of bacon!) look like they're needed.
First time I've used cast iron and it's remarkable how much heat they retain... they've been out of the oven for a good 25 mins and they are still far too hot to touch without gloves.