Crazy paving trouble

Some years ago I laid some crazy paving in my back garden. It was my first attempt and while the slabs are level and firm and on a good base I didn't do the filling in (grouting ?) very well. I used a dry sand\cement mix, brushed it in on a dry day and watered it with a watering can with a fine rose. I have since seen Tommy Walsh do the same but watering the troughs well before brushing in the dry mix. My cement between the slabs is very weak in places and starting to crumble away when I pull out any weeds. I've tried patch repairs but would prefer something with a bit of flexibility so it doesn't crack and crumble again. Is there anything else I could use to fill in the gaps ? Maybe something out of a cartridge ? Or maybe a flexible tile cement ? Thanks.