What pads are compatible? I have Tektro R320 Calliper brakes on my bike:

http://planet-x-bikes.co.uk/i/q/BCTKTR320/tektro_r320_brake_caliper

It's time for some new pads.

However, the ones on this appear to be all in one, bolt straight into the pad on the calliper.

If I look on wiggle, my usual online component supplier, there are lots of blocks for cartridges, or blocks like this:

http://www.wiggle.co.uk/swissstop-flash-pro-green-high-performance-blocks/

But not so many options that look like what is in the Tektro OEM.

How do I know what I can fit? what's compatible?
They look like standard Shimano-type cartridge brakes. No need for new blocks, just get a set of pads.
I recently got these and they are really good - far better than the standard shimano ones: http://www.wiggle.co.uk/kool-stop-dura-aceultegra105-pair-of-cartridge-inserts/

Go for the salmon if you have non-carbon rims.

EDIT: I seems your brakes do not in fact have cartridges but the brake pad is a single piece that connects directly to the caliper arm. The dura-style cartridges (like the ones you linked to) will fit on your calipers and is a worthwhile upgrade. You will be able to more cheaply and easily replace pads and have a wider choice.