I am a practising barrister, I considered doing a masters but a) couldn't afford it b) didn't want to study anymore and wanted to get cracking. It's by no means a requirement; it won't necessarily help cover up a weaker degree score. As you've pointed out, 5 years is already pretty long!
I'd say do it if you are interested and can afford it (it's unlikley to hurt your application), but it's definitely not necessary in most places. Quite a few juniors will have them, but some of that is correlation rather than causation - people who go to the bar as a rule often enjoy the academic study of the law, and therefore want to pursue it further / have more time at university etc, and it's not a career where a masters makes you look like you don't want to grow up or anything like that.
PS: If you've still got to do a law degree, don't think about masters now. You may study law and decide you hate law generally, or that you just don't want to study it longer.