The one you need is not stocked at maplins.
It is a KNB1530/1560 0.68uF that Drayton use depending on the year of manufacture. You can buy a direct replacement KNB1530 0.68uF on Ebay from seller al222b.
If you open your reciever and have a look you can compare the picture and capacitance to the one on the listing. The 0.68uF is the main factor to consider but it must also be 275VAC, Class X2 and the dimensions need to be the same so you can fit it easily. I found terminal length is important too. The ones on ebay were a perfect match for the one in my reciever. I got one extra as a spare because it will happen again in another few years and I didn't want to hunt around for them again.
I wasted months not fixing mine and turning it off an on again. After I changed the capacitor, which only took 10 minutes, it has worked perfectly ever since.
By the way the capacitors don't blow. They are used to drop the voltage but as they get old they become defective and the voltage drop increases and then there isn't enough voltage to operate the relay to switch the heating on.
Hope this helps you get you heating fixed.
Tom (Leeds)
February 2012