The "Charter Flight Centre" - any good?

They've been around a long time if thats an indication. Should be OK, at the end of the day they are only a middle man booking you on the charter flight with whichever tour operator.

Avro are good, but will only return results for Avro charter flights, not the competition. Other good sites are Flightline, Airflights, and flydeals, they tend to cover a good selection of tour operators. Moneysupermarket also has a fairly comprehensive search.

You could also try going direct to the tour operators who are selling the charter flights:

Thomson (and First Choice, Portland Direct)
Thomas Cook (and MyTravel, Direct Holidays, Airtours)
Monarch (also sell scheduled; but include Avro and Cosmos)
Libra Holidays (fairly small)
Olympic Holidays (for Greece)

There are probably other smaller operators I've forgotten but you get the idea.