I’ve been looking for something similar, but for 180mm – if you can’t find an actual Formula adapter, shimano or magura ones are supposed to fit. Or you could maybe try these guys: http://stores.ebay.co.uk/International-Bike-Disc-Brake-Pads

oh and for bargain Ortlieb-a-like dry bags, check out Crosso
They’re Polish, and the Zloty/£GBP exchange rates are pretty good

Last year I tried a two-wheel trailer and really didn’t like the handling, so went for a Weber Monoporter (Edinburgh Bicycle have em on sale at the moment).
It’s really well designed, light and foldable (unlike the BOB). I’ve used it for a couple of carless Merida weekends, a long tour in europe, and closer to home it sometimes comes in handy for getting bulky stuff back from shops (so far: microwave, small fridge, new wheels from CRC… etc)