In reply to DaveN:
I got a Hamax on my winter/commuter bike (the one that can recline a bit with the knob).
Daughter is 20 months old (and lightish)
She wears a keds helmet I think ( I had to keep looking for the smallest one.
I like the design of clips on this seat (she can undo her phil and teds pushchair clips).
I think you can buy cheaper versions from go outdoors (but i haven't checked them out - i.e. clip design)
For sure the centre of gravity is higher - but you get used to it - you can practice with a weighted rucksack in seat.
For me being 8 and a bit months pregnant - the hardest thing is getting my foot over the cross bar (where I would normally swing it round the back there is a seat in the way!)