It a completely personal choice. My kids are similar ages to yours. They are lovely, I really love having children and the best choice we made was to have them. But I wouldn't want to go back to the baby stage now. You said
I do think we need a breath of fresh air
Life has become boring and we have a very set routine right down to the minute. I feel like we need to be young again
Would having another baby help with that? Surely having a baby/young children means you live more in a routine, I know we did. Now they're older we can decide to just go out for dinner or the cinema with or without them without having to plan childcare and pack a bag. Holidays, weekends away or just a shopping trip require less planning too.
You also said
Nappies n bottles n potty's n playgroups will get on my nerves but my teenagers will be willing to help as much as needed as they love sorting out their baby cousin when she comes to visit
I'm sure your older two woukd love their sibling if you go ahead but I really wouldn't be planning on their help too much. There's a big difference in fussing around a cousin who visits than helping out with a sibling who lives with them. As much as they'll love them, your 16 year old is likely to be off doing his own thing a lot and your 12 year old will likely change a lot in the next couple of years as well.
Sorry that all sounds really negative, I didn't mean it too. Just that it sounds like some of your reasons for wanting another baby are that you want a bit of a change. If you sit tight for a few years, your kids will be completely independent and you can do lots of things which haven't been possible because of having school age children. Your children are old enough now for you to maybe not have such a routine so maybe plan a few different things to do.
Good luck if you do decide to have another baby though. I don't know anyone who regrets having children so I'm sure it will all work out either way.