Welcome, Andrew. You should also look at the "LAB Lounge" sub-forum as your set up would be similar to a pub (or so I imagine). UK garage size -- would that be about 3.5m x 5.5m? How big a space for a garden party?
I respectfully suggest that one subwoofer (perhaps a JBL PRX 718s) and one top (like a Yamaha DSR 112) might be a good starting point. Both are active (self-powered) and have pretty darn good DSP and limiting. The sub has a good cross-over, so you won't need any external processing. I am guessing that is approximately your budget for new or very lightly used. Make sure that you can get the top speaker elevated and pointing down to direct the energy down to where your listeners/dancers will be. You can sell the gear you already purchased and that will help with the budget.
It may sound like hearasy to not run a stereo system, but a single good speaker will sound way better than a pair of so-so (or worse) speakers. There are lots of threads discussing that point, if you are interested.
Good luck, and have fun.