Its worth a spacer regardless for safety reasons and just because the tyre clears when cold and stationary it doesn`t mean it won`t foul in use.
My 1990 Cooper has 8 mm spacers with longer studs as my rear shocks were quite chunky but if you have standard sized rear shocks the 3 mm spacer will be fine and if JCG supplied it with the handling kit back in the day its because it was necessary.
As for the cap http://www.minispare...sic/GRC207.aspx is the one you want as you have a small expansion tank. Its probably about the same price too as MS will post small items quite cheaply.