After doing some research
After doing some research online I bought a Kryptonite Series 2 D-lock and cable from Parker International for under £20. It’s solid and well made, and when I checked the spec those are nearly as hefty as the ones costing nearly twice as much. The moneyback/theft protection thing is full of conditions and only effective for the first 12 months, so shouldn’t be a factor. I considered OnGuard, which appear very similar, but read a number of forum posts about stuck keys/mechanism and that put me off.
Cable locks are OK for low security areas but are very easy to break – in some cases with a pair of pliers or garden secateurs. The reinforced cable type, with linked metal outer sleeve, would be better but they’re nearly as heavy and expensive as the D-locks. Heavy chains look tough and weigh a ton but are more easily broken than D-locks because the tealeaf can get it or the padlock on the floor and use leverage to cut a fat chain in seconds (if equipped with bolt croppers, as too many are).
BananaDrama, I’d look at Krypto Series 2/Evolution and Abus/Trelock equivalent for a good D-lock, if you have a nice bike. Sold Secure’s bronze, silver and gold (http://www.soldsecure.com/Leisure.htm) are a guide to relative resistance but don’t rely on them. I take a cheap cable lock if I’m popping into town for something but the D-lock if I’m leaving the bike for any length of time. Any lock is better than none, and in a row of bikes a thief will go for the easiest first.