What sort of price range are you looking at, are you wanting 30 days for 8 cameras recoring constantly or will you be using motion detection? Motion detection will only record movement so theres not usually much point in recording constantly.

Do you want it to have a cd writer to burn video to, or can it be networked to an existing network and saved and ripped using another computer. Or do you want to buy another computer to get the video off the dvr hard drive when using for evidence.

how many frames per second do you want to be able to record, if you wanted 25/30 frames per camera possibly all at the same time then you need something capable of 200fps (8 x 25). You would need a large hard drive too.

Depending on your specific system requirements, you could be ok using a 50fps dvr if you dont have much activity on most of the cameras alot of the time. It all depends on what you need, are any of the cameras ptz?

The system is located in a pub - therefore all of the cameras would need to be recording constantly. There are 2 cameras outside in the beer garden, which at this time of the year could be set for motion detection, but come the summer months I would want them also recording constantly as well. None of the cameras are ptz.

There is another computer nearby (on a LAN) so could look to use that to burn CD's if/when needed.

What would be the lowest fps I could get away with? Ideally I would have all cameras on the highest fps possible, which may well mean a 200fps dvr with large hard disk. Using a 50fps dvr would give 6.25 fps per camera - which presumably would be pretty useless? But what's the price increase to obtain the higher fps (and also a larger hard disk), and would there be something in between that would be suitable?

With regards to price ideally the lower the better, however I do understand you get what you pay for, especially on the reliability front - so would I probably be looking up to about £400 (+VAT) - but is this realistic?

If it looked like costs were escalating because of the requirements, it might be more economical to just replace the existing mux - any good deals around at the moment?

What I would suggest you do is have a look around and see what you can get for £400 in the line of 9ch dvr's then post the links to the specs here and we can give you more of an idea of its suitability.

depending on what the dvr uses for image compression depends on the size of the hard drive you may need, also taking into account of the units max fps.