THE Landmark Trust has published the 1994 edition of its handbook which lists more than 150 'worthwhile buildings' available for holiday letting. These include a Georgian folly, a late 18th-century cotton mill, a converted pigsty and a Gothic temple. A week in the pigsty, for example, which sleeps two, costs pounds 301-pounds 502. For a copy of the handbook send a cheque for pounds 8 to The Landmark Trust, Shottesbrooke, Maidenhead, Berkshire SL6 3SW (credit card orders 0628 825925).