27 years ago or so when I was a child I took piano lessons and got to grade 3; since then I've not played.
I've decided that I want to start again and have bought myself an electric piano. In the past few days I've realised that I can play all of the major scales un-sighted with left and right hands with the correct fingering so something still remains in my brain at least (though synchronising between both hands is tricky)
I used to be able to sight read music but as you can imagine I'm somewhat rusty.
What's the best approach now to regain some of my skills? Should I get a teacher or are online courses sufficient? I've had a look at the android app flow which looks good.
You do not need a teacher to relearn how to read music or get used to coordinating your hands again. Start with music that is easier than grade 3, then increase the difficulty gradually. You will find that you will go through this rather quickly because you are relearning instead of learning from scratch. Once you feel comfortable with reading the music and playing hands together, I would find a teacher to help you with technique. I don't think that is necessary immediately, but be aware that you may have forgotten some technique and will need to make some adjustments to your playing.
So you reached grade 3 in the past. Does that mean you still have the books from grades 1-3? Because then I would suggest that you play through those books from start to finish. I think that will help a lot in order to regain your skills.
Then take it from there. Play some more pieces at grade 3 level in order to consolidate your skills (you can find grade 3 books via Google). Also, get hold of pieces at grade 4 level in order to expand your skills.
Bela Bartok's "Mikrokosmos" is also a very interesting work you could pursue.
If you still have music from your past that you learned, you will find it much easier to relearn it, as you have already played it. Playing some of your old pieces will be fun as well as good practice for relearning. You can then find some new pieces that you might like to play, but I recommend starting off with songs that are a little easier.