Have a good search on this site we have discussed this subject inside out and the conclusions in general were that there was little or no real gain with the thermal spacers in this country, especially if you have the one with the valve and forget to turn it back on.
Horsham on this site supply their own make of this, because after all are they are only an aluminium spacer when all's said and done, and they were doing a group type buy a few weeks ago, Jez would probably do you a good deal without any need for a group buy.
I have one and they do make a difference in air volume , one of the main ingredients in improved performance.