POD sell rucksack side pockets (attached via rucksacks compression straps) but they're fearfully expensive. Anyone know of cheaper ones?

Try some military/surplus stores. Most Bergens and patrol sacks have detachable side pockets, or at least the facility for them via zips. The side pockets are about £30 for two, and i guess you could rig up some kind of system for attaching them. Bit of a colour clash though probably...

Aiguille Alpine Equipment
Any good?

(Ah, perhaps no cheaper than the POD ones, but bigger)

Decathlon sell lots of add on bits.

when i saw this thread, I knew i had seen what you wanted, but where and by whom???
I searched the depths of my memory. and then hey presto:

http://www.outdoorgb.com/p/deuter_external_pockets/

hope these will help

Thanks for the comments in general and to Morph in particular. Those Deuter ones are much cheaper. Don't mind paying 12 quid.

Lowe Alpine do them as well £9 and £12 inc delivery:

http://www.jackson-sports.com/Models.aspx?ModelID=2904

Thanks for links above. I didn't even come on here looking for side pockets. But the Deuter pockets will fit just right on my Berghaus Munro. I had lost the original small Berghaus add on pockets ages ago, and these are about the same size. I Might even add some straps to the front of my 35 litre bag, so I could put on on the front, for gloves/hat's etc.

For a cheap DIY solution you can use a spare compression stuff sack. They have straps running along their length. You can thread the rucksack's compression straps under the bag's straps to support the bag and hold it against the rucksack. The bottom of the stuff sack can flap about a bit so I generally use a bit of paracord or whatever to anchor it.

KarrimorSF (the military branch) do them, but they may be the PLCE zip-on ones only.