Alex that's totally normal for bf, it's because it's all goodness there's basically no waste to come out and once their digestive system develops is totally normal to go that infrequently.
We started at about 20 weeks I think too. Started with veg purée as I was told that if you start with fruit you can develop a sweet tooth. M loved baby rice, I think because it just tastes of milk, and I found it really useful when introducing new flavours (eg he refused broccoli purée but happily ate it mixed with baby rice, now will eat it on it's own) and to get some carbohydrate into him. Agree re flavoured ones, no need, just mix it with your own foods.
You're supposed to introduce one new thing a day so if there's a reaction or allergy you can pin point it.
Big faves here to begin with were root veg and sweet veg, so he loved puréed sweet potato, butternut squash, carrots etc. I cooked up batches of each then started to mix then together, and then with other things eg salmon or chicken.
Now he likes pretty much anything and will happily eat a wide variety of things, from spag bol to fish pie wil curly kale. He's got much better taste than my DP! 
At the airport drinking wine, waiting for our flight out to France. Pop over to say hi hazle! Although not long until we get to meet you. Have we persuaded you to come down for that Cavort? And are we all leaving babies at home, or bringing them?!