Don
I did a little further research they are both readily available in the UK.
Use them as a top dressing, so the approach that I was going to take by using them as layers in a compost bin would work as well.
It will make both of them go further and it will ensure a more even spreading of all the minerals, trace elements and organic matter needed for healthy growing.
You can find Rockdust here:
http://www.dtbrownseeds.co.uk/seeds-plants-gardening/62247/rockdust-x10kg?gclid=CLn2i5OWtqsCFQMPfAodMGiScA
Or here: http://www.greenfingers.com/superstore/product.asp?dept_id=200437&pf_id=LS3848D&co=fr
Which has information on application, advises not to add lime.
As for Biochar try here for discussion:
http://www.allotments-uk.com/forum/topic.asp?TOPIC_ID=9551
And here for suppliers:
http://www.harrodhorticultural.com/HarrodSite/pages/product/product.asp?prod=GFE-003&cookie_test=1
Its a matter of doing a bit more searching to find larger 20Kg bags as it works out cheaper and for one suspect that 20Kg of either or both would be enough on a rolling basis to keep your soil healthy.
The only other thing I would suggest would be test the pH of the soil on a regular basis to ensure it is neither too acid or too alkaline.
As my soil is still a bit claggy, lacking a lot of drainage and still in need of organic matter/humus then the addition of Rockdust and Biochar can not effectively do it any harm.