On the bench is a Tamiya 1/48 Mosquito B Mk IV.

I'd run out of Xtracolour X117 (Interior Green) and thought i'd buy a tin of Humbrol 78 (Cockpit Green) as it was just easier at the time.

How close a match to the original cockpit interior colour is Humbrol 78? Tamiya want you to mix a few colours together to achieve what they think is correct, is it worth doing that instead?

78 is a pretty good match right out of the tin, no mixing required.