I agree - decking oil. I bought my house last year and the previous owner used shed & fence treatment on the decking - what a mess! It's not tough enough to take foot traffic and just wears off in patches.
You can buy block paving sand in Wickes. It's basically fine, kiln dried sand in 25kg bags. They also sell a patio grout which sweeps in and is self setting - this is the longer lasting but pricier option.