Hi Lewis' Mum.
I found that my 3rd son went from being a fantastic sleeper to being unsettled at night around the 16 week mark. I do honestly think it was due to him start to get hungry, that being said I still held out til he was about 4 1/2 months and he was given some rice cereal morning and night, slowly increasing to small amounts of vegies and fruit. I maintained this until he was 6 months old and then introduced all the foods to him.
I no I go against the majority of withholding solids untill 6 months but I have read several different reports on the benefits of slowly introducing solids from 4 months, so I wasn't concerned with that. It too another couple of months for his sleep to return to going through the night and now he is just a champ and sleeps through 11 hours at night.
Good luck, you will ride through this phase.