Benjii... or for all those who are interested... Been advised that this would be the cheapest (perhaps not easiest, as it would involve manual labour):
1 of 100 X 100 SYSTEM 1 WEDGE GRIP REPAIR SPUR METPOST (http://www.tottontimber.com/system-wedge-grip-repair-spur-metpost-p-2413.html)
1 of 3.0M 100 X 100 PRESSURE TREATED POSTS (http://www.tottontimber.com/pressure-treated-posts-p-406.html)
Cut old post down at ground level + whack spur into ground + insert new post into spur + attach gate = job done for £25.00 and a bit of a sweat on.