Yes, usually it makes sense to purchase a 'bundle' including a charger and 4x (or more) of the battery size(s) you need. Some older chargers are only suitable for use with Ni-Cad batteries, not the newer Ni-Mh ones, and, conversely, the newer 'fast'/1-hour chargers aren't safe to use with Ni-Cad cells (tbh, I'm not too keen on these ultra-rapid chargers anyway — they heat up the batteries something terrible, and I'm sure they must reduce their life...)
7dayshop often do such 'bundle' offers — see
here, for example — and with the £3.95 flat rate shipping, it would obviously make sense to order all your extra batteries at the same time...
You can sometimes get even better value on eBay — I can highly recommend [broken link removed]. I once got a faulty charger from him, and when I contacted him he immediately sent me a nice apology and a replacement unit, plus four extra batteries,
without requiring me to return the faulty one.