1. The locknut tool needs a number 22 spanner, which is bigger than any I own. Thanks to IWJ for lending me an adjustable spanner that did the job.
2. Get a spare rim tape. The old one might have perished and split around the valve hole. A new rim tape would be better than bodging the old one with gaffer tape.
3. Make sure your nipples are well greased (oo-er etc.), or you'll round them off. Oops.
4. If the man in the bike shop gives you an old wheel for free rather than selling you a new rim, thank him profusely and show your gratitude by buying an inner tube with a compatible valve for the size of hole in the rim. Otherwise you'll spend hours building and truing your nice new wheel and then discover that the Schrader valve from your inner tube doesn't fit through the narrow hole in the new rim. DAMN!