@Iniq: I just realised that in writing the above posting I have assumed that a gilt fund would necessarily be of the AUT/OEIC type. That's not quite true.
There are some ETFs that track gilt indices, so you could invest in gilts indirectly that way too. The mechanics of buying and selling them are the same as for direct equities. In most respects they behave like an AUT/OEIC-based gilt fund would. The differences are (1) you will always pay buy/sell commission (2) there will always be a buy/sell spread (3) you may be charged a custody fee, if your ISA operator does that for direct equities.
I would not use this route because it seems to combine the disadvantages of both of the other two methods. That's probably why I overlooked it first time round.