Thank you for the input everyone. I have used their commission bidding in the past, and have won items using it. There is a gun coming up in the next auction that I have been looking for, for a very long time and will be bidding on.
Like Toby said, it seems like your commission bids always seem to end up at your maximum. This particular gun will most likely go for a very modest amount, and I would hate to place a very large commission bid on it knowing I might be wasting several hundred Ł if they automatically place my maximum commission bid.
Maybe the best plan of attack is to place a modest commission bid, and then try my luck with Artfact, if my maximum commission bid happens to be exceeded.