It depends what type of bulb you use, and what type of bulb depends to an extent on the size of your tank. If you have a big enough tank you can use a combined heat/UV lamp (MVB), but they can easily overheat smaller tanks and in that case you'd have to use a separate UV and a spotlamp.

Clamp Lamps are ideal for holding spotlamps, just get the smallest (14cm)one and fit an ordinary 40-60W reflector spotlamp (R63). They can also be used for compact flourescent UVBs, but the white enamelled inside isn't as good at reflecting UV as a polished metal one would be, if you can find a polished reflector that would be better. IMO the Exo Terra Repti Glo 10.0 Compact is the best UV lamp to use.

For bigger tanks though (say 36"+) you can use a 100W MVB instead, again you could use a Clamp Lamp to hold it, but because these bulbs get very hot you need to use a bigger one to ensure sufficient airflow around the bulb. I think the T-Rex Dome Reflectors are better for these bulbs, or a similar lamp holder sold for use with chickens is a cheaper alternative and works just as well.

You need a water heater for now as the water temp should be aroud 25-26°C until they are a year old, after that the heat from the lights, and background heat from the room will be sufficient to keep the water around 20°C, which is plenty for juveniles and adults. An ordinary aquarium heater with a built-in thermostat is fine and cheaper than a mat and stat.

I wouldn't recommend a filter with a heater, they are quite expensive and after a year you don't need the heater anyway, better to spend your money on a bigger capacity filter. Take a look at the larger filters in the Eheim Classic range, big, simple, and reliable.

The tank is 36in*15in*12in

Something like this would be ok for the heater?

Clearseal Heater 100W 90 150 Litre Online Pet Shop UK

And something like this for the filter?

Eheim Classic 2213 Plus External Filter

The heater is perfect, I'd go for a bigger filter though as that one's a bit small for a 3' tank with 2 turts. Remember it should be rated for at least twice the volume of water, or more if you have more than a couple of turts.
I use a 2213 for a single 2" turt in 100 litres of water, and while it might cope with your two for now I doubt it would still be coping in 12 months time when they are twice the size.
The 2217 would be better, it'll last a good couple of years, and when you eventually upgrade to a bigger tank you can simply add a second one. You can get them a bit cheaper from Pet Supplies, Pet Food, and Pet Products on Sale Now at zooplus.co.uk, I paid just under £60 including delivery for mine with a new customer discount.

With a tank that size you could also consider a 100W MVB, provided you can mount the bulb around 15" above the basking spot it should be OK.

Does the heater need a guard etc or would it be ok as it is?

I'm guessing that the tank will be between 1/2 and 2/3 full of water for the heater to be submerged properly

Any recommendations on where to get the lamp?

Found another filter on that site you gave me - Fluval 205

To be safe I would use a guard, little turts tend to wedge themselves in behind heaters!

Fill the tank as deep as you can without them climbing out, you can never have too much water, more water makes the filter's job easier as well as providing more swimming space.

You can get a 100W Mega Ray from Southwest Exotics Home page., although they are closed for holidays until next month, Kimbos Reptile World also have them, or RFUK member Rachel has them for sale in Equipment Classifieds.

Check the rating though! It's only rated for aquaria up to 200 litres, so that's 100 litres or less for turts, it's not even as good as the Eheim 2213 in that respect. Eheims are much better filters too, ask anyone who uses them.

Can you use anything for a guard or are there specific ones you can get.

I'll have a look in the classifieds now.