I've decided to go for SF40 insulated blanket on 50x 25 battens with upvc cladding on 20mm thick battens. To get around the leaky problem I'll fit large L angle wedged to the right angle, sealed onto the ring beam. Then drill vent holes, at intervals just above the angle, through the ring beam with sealed in vent pipes to drain outside. Hopefully it should work, after all it's only the winter and southeasterly gales that cause any problems. I can live with that. Comes in at about £350 to £400. I worked out if I lived the next 20 years just heating would cost £2000, a bit cheaper than £7000 quoted for a roof and still have to heat it.