[1] Heriot Watt Univ, Dept Math, Edinburgh EH14 4AS, Midlothian, Scotland
[2] Heriot Watt Univ, Maxwell Inst Math Sci, Edinburgh EH14 4AS, Midlothian, Scotland
[3] Univ Newcastle, Sch Math Stat & Phys, Newcastle Upon Tyne NE1 7RU, Tyne & Wear, England