I'm sure you got it right Nige but a lot of people don't.
A length of pipe doesn't provide room for expansion
within it at all. If there's non return valve, you need an expansion vessel no matter how long the pipe is after it.
The confusion comes about because of the need (if no EV) to allow the water to expand back along the pipe and NOT then go out of a cold tap.
The instructions are often ambiguous. If you work out the possible expansion on 15l of water (allowing say 4%, = 0.6 litres) you find that the length of pipe they talk about IS that volume. The instructions often seem to imply that the length of pipe will magically expand to accommodate the extra water.
It's normal to "get away with it" if there's a float operated valve (wc, loft cistern) on the supply pipe. The pressure goes up, and the water in the pipe pushes past the valve.
Often see small unventeds installed wrongly though, with no expansion provision. Can only assume that the expansion in the normal range, say 10 - 60 ºC, only say 2%, is taken up by distorting the heater vessel
. Or dribbling down the prv pipe - if there is one!