Unless the pc is in constant use, perhaps as a web server, there is no reason to for it to be switched on 24/7. To keep it on all the time is just waste. Many will tell you the hardware will be adversely affected by being switched on and off, but unless you switch on and off repeatedly, rather than once or twice a day, this is just not true with modern hardware.
You will probably find that £8/£10 per month is about right. Also take into consideration, all the additional that you may have on also, like printers,scanners,cameras,modems etc. Only small wattage, but it all adds up at the end of the month.
I was reading a university report recently, covering items that are left on in the average household and small industrial premises. Made very interesting reading, regarding costs and safety factors. Just tried to locate the item,for some facts, but it looks like I have recycled the publication.
In the average desktop setup the monitor will use a little more electricity than the computer 500w i think therefore just over 1£ per 24hrs. although the computer may have say a 450w psu it will not be using this much all the time so cost will vary