Definitely get a drs letter. Easyjet made us chuck our bottle at Xmas despite our pleas. Never had a problem before - other than being made to swig it in front of them once... that was a nice mellow flight for mummy so we hadn't bothered with a letter! Will do next time!
Your best bet is to decant an 'emergency' dose into a small plastic bottle for the flight and pack the larger bottle in your luggage. The problem is not the liquid it is the quantity. We do carry a GP letter but also carry Epipens. Try here for small medicine bottles: www.alwaysreadthelabel.info
If worried, I used to decant into an emergency bottle like tinytalker suggests. But have flown at least twenty times with Ds and his piriton and never had any difficulties at airports. That said, I've never flown with easyjet! Didn't realise you could get small bottles from chemist!
Seconding (thirding?) the small bottle advice. We decanted some piriton into a 100ml bottle and packed the big bottle in hand luggage. We got the small medicine bottle for free from Boots - just ask as they have a supply of them. You can also buy dispensing syringes for a few pence if you need them - our branch of Boots keeps them in a drawer so you wouldn't know they are available unless you ask.
Thanks everyone. I decided to play safe & asked my gp for a letter which they had no problem issuing & also got small bottles from my local chemist & he put prescription label on them so think all bases covered