It all depends what you want to do with your money. It seems ample enough for one week.
Make sure you're not left with any Dinars when you return home. for you get a bad return exchange. Also keep receipts when you change traveller cheques, as you will need them to re-exchange
Life is very cheap overthere, as I said it depends what you want to buy.

You could go to Nabeul on a Friday morning, the weekly pottery market.
You can take the local bus for that, every half hour and it will cost you less than 1 Dinar (depending where youre staying in Hammet.
You could take the local bus to Tunis. Every hour. approx 3.500 Dinar.
And of course Hammamet itself.

Weather is still very warm, should be super.
It is Ramadan at the moment so check time-tables carefully. Inside the hotels it's ok for eating and drinking, but outside during the day it is less easy. They don't eat, drink, smoke from dawn to dusk.
Hope this info will help you a little bit.

gert-jan

You will still be able to eat inside most restaurants during Ramadan but will not be able to eat outside in the sun until after sunset, nor will you be able to buy alcohol outside the hotels, certainly not in supermarkets. £300 should be ample for a week unless you want to do many long trips - the 2-day Sahara trip for example costs about £70.

Take some travellers cheques in small denominations,ie,£20s,so that near the end your not having to change a large amount,or keep a £10 note,because at the airport if you have dinars left they take a huge chunk,and you end up having to give about 4 dinars for a £1, or near to that!!.Also once you have gone through passport control into dutyfree and cafe they dont accept dinars.