Sorry Dappled, you are right, and I was wrong, post towns clearly exist, and I have been able to ignore that because I have only ever lived in a place where the 'post town' (which could also be a city of village) and my location matched. However, they are even more misleading than post codes, which I suspect is why Royal Mail don't go out of their way to advertise their existence, given that all necessary information is contained in the postcode.
They simply say:
Next line can be used to name the local area or village name.
Next line should be for the town. Write the name of the town in capital (upper case) letters.
without any explanation of what they mean by 'town'.
There is indeed a subtext and it is:
"The address information we provide on our database allocates districts to sorting offices, but may not be an accurate guide to location and may infact be misleading. We have allocated the village of Tiptoe in Hampshire to the 'post town' of Lymington (6 miles away), although the closest town is New Milton (2 miles away). Also please note that while Tiptoe has a Southampton postcode it's actually much closer to Bournemouth.
While we are apologising we would also like to apologise for suggesting that 'Sewardstone Road, London, E4' might be an easy walk from 'Bow Common Lane London E3' or that Sewardstone isn't in Essex. We also know that London SE17 is randomly between SE1 and SE5, but please, can we not talk about that?
Yes, on reflection Queens Road Richmond TW10 and Queens Road Richmond DL10 are 250 miles apart, and that isn't really adequately conveyed by the address provided on our postcode database. Do please also provide county details. The world will still spin on its axis if you write 'Richmond, Surrey', it could be helpful it the TW or DL get smudged, and everyone knows what you mean.
To be honest to find your address we only need your house name or number, road and postcode. Everything else is additional to our needs and the "post town" information we provide could be confusing for anybody trying to find your address. You can reach Tiptoe without going through the Lymington Delivery office. We didn't really think things through before we sold all this confusing information to 3rd parties. Sorry."