Do you have to have brake pads replaced if you replace discs?

Yes - simple as that. Crazy not to.

If the discs need replacing they will be warped or grooved and the pads will have compensated for this. When you put them back on with new discs chances are they won't be parallel with the disc surface and so not work as well as they should. So new pads always, they are quite cheap compared with the discs and you shouldn't be charged any extra labour because they would be removed anyway.
On the other hand, if you just require new pads you shouldn't normally need to replace the discs. Audi insist new discs should be fitted when changing pads but that just seems like blatant profiteering.

Why wouldn't you? By the time you've taken everything apart to replace the discs it's only a small extra cost to replace the pads, then your brakes are good for a year at least.

yes its sensible to replace them both ...the disc are not too dear ...unless you buy them from ford ...you can buy a pair circa £40

Yes, it's a good idea to do both at the same time.

It's a very good idea to install new friction pads on your new brake discs.

Why not, since you're already in that deep on the job. If there's lots of meat left on the pads and you need to keep the cost down, you can reuse the same pads. Here in the US, I've been paying about $25 each for new brake disks and replacing two rotors takes me about 1/2 hour.

Yes. The pads will be the contour of the old rotors.
The rotors are not expensive.
Any garage will quote.

A better question, why would you even think of replacing the rotors (or discs for what that means) without replacing the brake pads. But I question what you really mean by disce.

I wouldn't even think about it unless they were in like new condition with very little wear.