I'm about to purchase new brake discs online and want to make sure I'm buying the right ones.  When I enter my reg on halfords or eurocarparts, are the discs that come up guaranteed to work?  Is there anything else I need to know when buying brake discs? 

I would never rely on that for 100% certain.

The least I would do is measure the diameter of your existing disc.  I would also check to see if your discs are vented (air gap between the two surfaces) or not.

It always seems to be sods law that when I am buying parts I have a car built at a time when the makers were changing over parts eg. from Girling brakes to ATE brakes etc.(I don't know if there was such a change of Fiestas that's just an example of the kind of this you sometimes come up against)

Halfords just buy their parts from ECP btw lol...

The ECP brake checker is pretty useless tbh, it'll often show you all the available options for the car model, without even reducing it to just the correct engine size which is usually what denotes brake size.

Best thing to do is measure across the diameter of your own discs if more than one size option is coming up. 

It can be difficult to find what you need as i found out having just rebuilt 3 front ends from various models. A lot of online info is, well, wrong.

There are so many variations with the fiestas the only way i managed to get it right was by phoning various suppliers and emailing pictures of the parts especially caliper bits. The discs are a little easier i think they are 263mm either solid or vented for most basic setups the st go to 278mm.

A bit of browsing, talking with suppliers and patience will make sure you get the right bits. Good luck.

Haynes Manual would be useful.

Textar brakebook might point you in the right direction, although it may list multiple parts, look at the dates. Eg For vehicles up to 2001, for vehicles from 2001

https://textar.com/brakebook/