You can book internal flights online with the following airlines:

Thai Airways
One- 2 - Go
Asia Air

all will be much cheaper than you have been quoted, but the last 2 are budget airlines

Take a look at Air Asia and Bangkok Airway's son has paid 38 pound return Bangkok/Chaing Mai with Air Asia. I also have flight's booked with them for April at a cost of 10 pound return KL/Penang and 33 pound return KL/ Borneo.

Air Asia are Ok...baggage allowance is 15Kg. I would build in plenty of leeway though for flights that may have to connect with an international flight back to UK as they have been known to get delays or even cancel. Our flight with them back from Krabi to Bangkok copped a two hour delay.

Might be too late now but...

You have to use Bangkok Airways for Bangkok - Koh Samui - Chiang Mai as they own Koh Samui airport & don't let anyone else fly from it

And they have 1x non-stop Chiang Mai to Bangkok a day.

But £230 per person for those 3 flights? Doesn't make sense - should never be more than £150 maximum. I wonder if they are making up for an error somewhere else and trying to recoup with these flights??

Book yourself at http://www.bangkokair.com/ you can get all 3 flights here. Leave plenty of time to connect with international flights. And if you book Air Asia make sure that any potential cancellations won't leave you missing your international flights - as you'll be responsible for paying for new flights!

thanks for all your replys, in the meantime we have contacted ATOL and Trading Standards and they say the travel agency cannot charge any extra as we have written confirmation of the flights and already paid a deposit. I'll leave it in their hands for now.