Finally got a chance to get out to Wilson & Bradley to get some runners.

I got the Blum Tandembox 500mm runners. interestingly, my initial quote when I started this investigation was ~ $96+ per drawer (ouch).

Wilson & Bradley quoted me ~$64 per drawer (runners, plus 2 x catches required).

When i got to the counter and checked the price - yep, ~$64 per drawer. When I told the guy I wanted enough to do 14 drawers, the cost came down to ~$32 each (I don't have the exact figure on me).
He said they are supplied by Blum in boxes of 6, so an order of 6 or more will get you a reduced price.

I guess it pays to buy in bulk.

I hope this info helps anyone else looking to buy these runners in the future.
TN.