I found this on the internet:
Apparently they dont make it anymore and by the looks of it these guys had a custom chip made. Cloning one of these to write to SD Card would be the way, but where to start?
I own one of those - bought it when it first came out (and quite impressive for what it did at the time) - but unless you have a rich investor or something else up your sleeve (that, or a ton of time on your hands, as well as being an expert in FPGA/VHDL design) - you won't be cloning it.
If I were you, I would start by going back about 20-30 years, and looking at old BYTE magazines, Steve Ciarcia Circuit Cellar articles, and other old computer magazine articles (and books) from the 1970s and 1980s on "video digitization" to get an idea of what is going to be involved just in decoding a composite signal (and you haven't specified PAL or NTSC; but since you are in the UK, I assume PAL?) - not to mention capturing it.
Once you have a handle on that, you might want to then look into the Nootropic Design Video Experimenter shield (and code - which is based on the TVOut Library) to learn how it works to capture a low-res B/W version of the signal (color is another thing altogether):
Ultimately, you're not going to do what you want to do with the Arduino by itself; you might be able to use it for control of some chipset meant for this purpose (good luck finding and purchasing that chipset, though - I am sure such a thing exists, but it probably isn't sold by 1s and 2s for hobbyists). If you wanted to do this "directly", you'd be better off using a small embeddable PC board (ARM-based, most likely, running Linux), and using that (along with a composite to RGB analog conversion IC, running that to an external or on-board ADC channels) to write custom software to digitize and store the image to your SD card. Even that, with the simpler hardware, is going to be a software challenge like you wouldn't believe (well, you'll probably understand once you learn about how it was done in "the old days").
I will say this - if you are going for PAL composite digitization, it is supposedly simpler to digitize (and generate) a proper PAL signal than an NTSC signal (that's all relative, though).
Good luck with your project. :)