Piriform, an internet site where you can download their excellent Recuva software for free.
I filmed two buzzards fighting earlier this afternoon and I was deleting bits of unwanted footage from my SD card while not paying attention to the job in hand, needless to say I deleted part of the footage which I did want to keep (see below).
After cursing myself for being such an empty headed idiot I remembered reading an article in Computer Active magazine featuring Recuva data recovery software. So I typed Recuva into Bing and hey presto, I had found what I needed. It is only a small programme and totally free, but they do a paid version too with 24 hour technical support included. They also accept donations from people who only needed the free version but who would like to show their gratitude for recovering important lost data.
I cannot recommend Recuva highly enough and just in case you ever do accidentally delete a file which you need to recover it is worth remembering the name but bear in mind it is unlikely to work if you overwrite the file. So as soon as you realise you have deleted something important, you should set about recovering the lost data asap or it will probably be lost for ever.