OK so I am sure this question has been asked 100 times before on here but I cannot find where, so I was just wondering if somebody could tell me which relay I should use to turn on a 240v 15w heat cable via the 5v digital port on the Arduino Uno?
Thanks! 
Your best bet is a "solid state relay".
240V, 15W, not a lot of current.
So, you could get one of the "in line" type,
http://opto22.com/site/pr_details.aspx?cid=3&item=MP240D2
a small footprint and lower cost
compared to the high-current, screw terminal type
http://opto22.com/site/pr_details.aspx?cid=3&item=240D3
Those are just for on-off operation, not proportional control or any of that like.
Hello, I can suggest using a relay board such as this one (click the image to go to website)
It's much cheaper than a solid state relay, as you can see... On ebay you can find these for like 5$.
You can try to find similar board with only one relay... But the second relay here could be used in case the first one doesn't work anymore
Also, a link that can help you: http://arduino-info.wikispaces.com/ArduinoPower
guix:
It's much cheaper than a solid state relay, as you can see...
Depends. Shop around, surplus, in-line SSRs can be inexpensive.
http://www.bgmicro.com/REL1084.aspx
The Opto22 in-line referenced is $16.50, but a relay board like that pictured would cost the same or more.
Heck - with that low of current consumption (which is the important part here), you could probably get by with a simple reed relay and a small transistor driver circuit (under 5 dollars in parts); if you shopped carefully and know the resistance of the relay coil (or the current draw), and it was a 5V relay, you might even be able to power it directly from the Arduino (still need the flyback diode, though!)...
Thanks everybody for your fast and in depth replies I really appreciate it.
I am liking the blue one that guix has posted a picture of as it looks tidy and has screw holes which is handy as it is going inside a wooden enclosure. However is there one similar available with just a single relay due to space saving?
Thanks!
Edit: This one should do the job right? (http://www.ebay.co.uk/itm/1-Channel-5V-Relay-Module-Board-for-Arduino-PIC-DSP-Optocouple-SRD-05VDC-SL-C-/160881308759?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item257545f457&_uhb=1)
Yes, that one would do the job.
Perfect, thanks again everybody for all your help and quick answers as this another piece required for my university project ticked off the list 