+1 for the Topeak beam rack / slide on bag combo. Did 350 miles touring this summer with one (on my Roubaix…) and it worked a treat. I have to admit I swapped the carbon post for an alloy one for it, but other than that it was a perfect addition for lightweight mini touring.
I have a funny feeling that Halfords have them with 1/3 off at the moment, plus cashback if you go via TopCashBack too…
I'm thinking of using it for my commute on the other road bike, though I don't know what the water resistance of the panniers is like – it doesn't have taped seams or waterproof zips. I've used it for shopping trips and the side panniers hold pretty much a small carrier bag full of shopping each side – plus you can detach the panniers very quickly and take the whole setup into the shops too. The top part is insulated so good for a couple of bits of frozen stuff for a short trip home.