Tartan - thanks, I might go for four 40m (5g ) or two 82 (10g) balls to begin with, then. I've just realised that it probably be much cheaper to go for Anchor threads instead, which is slightly annoying as I've been emailing this seller on eBay.
Holice - yikes, that would take more than a bit of time! You're correct in that it has symmetry both ways and that a quadrant will give you the whole thing (ignoring the corner where I randomly tried drawing some Welsh scissors and then went off them), but with all the double lines and filling and so on, it would take forever to calculate. I'd rather overestimate thread, or take the risk that the dye lots might be a bit different.
Thankfully the yellow fabric is fine enough that I should be able to trace onto it. The plan is to draw it out in full on pieces of A1 paper stuck together, pin the fabric on top, and then trace it with Crayola washable pen. I've used them before and I'm happy with how they wash out. I have a grand piano, so I have somewhere to leave it flat while I'm tracing it (I won't be able to do that in a single session) and then while it's waiting to be basted. I'm contemplating starching it to make it easier to deal with at the tracing and basting stages, but starched fabric always creases in the Q-snap like crazy, so probably not.