I've brought some 20mm spacers, my dracos are being reconditioned atm so I can't test them. Will the wheel bolts be to long? Or will 20mm spacers work with the 20" Draco alloys?

 

Should be perfectly OK.
Some F-Type owners have had problems with 15 mm spacers, where the OEM wheel bolts poke through the spacers just a little too much for the shallow holes in the backs of the wheels (ie between the bolt holes), so they have had to either change to 17 mm spacers or grind about 2 mm off the end of each OEM bolt.
With 20 mm spacers and Draco wheels you should not have this problem.

 

That's great, thanks. Didn't fancy getting my angle grinder out to the Jag!