I think the departure tax is 700 baht. However, if you booked through a TO this may have been included in the price.
Assuming you are British then you don't need a visa. For visa regulations have a look here

Departure tax is included in the price of all plane tickets now. No need to keep any cash handy to pay the Departure Tax as in the past.

Thats strange as my tickets arrived yesterday and with them a letter saying i would need to pay the dept tax.

Chester - maybe they were issued before the departure tax was included - did you book them a while ago? If that's the case, you're not losing out, you just have to pay it at the airport. If you booked them recently, you might want to call the airline to double check.

booked two weeks ago so I will get in touch with them.

The last two years we have been to Thailand the departure tax was included in the airline ticket, you are given a 30 day visa on arrival, I try to work it so that we have 31 days, one day overstay is free, if you overstay by 2 days you have to pay for the first day and the second day, was 200baht per day, not sure how much it is today. (ask spud)

A Visa is not required unless staying more than 30 days. From my experience you fill out an immigration form on arrival and this is handed in again on departure. I visited Laos while in Thailand and if im not mistaking re-entry to Thailand puts extra days on your stay. You do pay for a visa to visit Laos (about 30 US dollars). I think there is a limit to this though so you cant just keep popping over the border and back. Best to check in advance though with the foreign office.