Do we need a visa?

No, you don't need a visa if you are staying for only a week.

We weren't sure whether to stay for a week, 10 days or 2 weeks, (thought we would see how we feel when we get there as we are lucky enough to be flexible atm, plus we are unsure which airport we'd prefer to fly back from,  Sofia, Plovdiv or Bucharest) so we haven't yet booked a return flight. Does this change the visa situation or should we just book a return flight?

Up to 90 days in any 180 period is okay, as long as you're not working while here. Presumably, you have  French residency?

@JimJ  We do have French residency, I just suddenly panicked, thinking they might not let us in if we haven't got a flight out booked 🤦‍♀️🤣🤣🤣🤣 I havent travelled far in the last 9 years, everything has changed so much that I've become a bit of a worrywort. 🙄

And defo not working, I need a holiday. It'll be our 1st ever holiday (combined with a bit of a reccy trip) and we've been together nearly 20 years.......

@ButterMyPaws19

Makes no difference if you have French resistance or not. You are allowed to stay in the Schengen area for up to 90 days out of 180. 

Bring your French residence permit with you when you come and be sure to show it ostentatiously to BG immigration at passport control; if you're lucky, they won't stamp your passport and the clock won't start ticking....

    Bring your French residence permit with you when you come and be sure to show it ostentatiously to BG immigration at passport control; if you're lucky, they won't stamp your passport and the clock won't start ticking....-@JimJ

It doesn't matter if they stamp the passport anyway if you have a residence permit.

@SimCityAT

That's indeed the theory - but that only counts in the EU country in which you're an official resident.  Unfortunately there have been a slew of cases where UK passport holders have been delayed, subject to unfriendly interrogation, or even refused entry, by officious EU immigration officials because someone holding an EU-state residence permit has had their passport stamped by an ignorant jobsworth in some other EU state. It's better to be safe than sorry down the line because your dates don't match up, especially if you got a stamp entering a country but not when leaving it.

@JimJ not it doesn't, I've had enough meetings with the embassy and ambassador over Brexit.

You are protected by the WA.

Again you're talking about the theory; unfortunately many border guards don't know what's in the WA (and probably don't care..). It's quite common for people to get grief - and knowing that you're in the right is very cold comfort. Many expat forums advise members to carry copies of various bits of legislation etc in the local language but even that isn't a sure-fire solution to a bloody-minded git in a uniform. I've several times found myself arguing the toss at Sofia airport with a border police bod who's insisting that ALL UK passports have to be stamped; unfortunately the 90-day rule still applies to holders of BG residence permits when they're visiting other EU countries. Luckily, I have an EU passport to trump their insistence but you're certainly right that these people should know the details of the WA rather than just making it up as they go.

Don't forget to swipe some Ferrero-Rocher when you're hobnobbing with the Ambassador...