The so called best people at this tried to build at the Government Watford Building Research Establishment and they failed.
This type of work is probably best done by you, where you can give it the time to get it right.
Polyurethane foam is probably required to fill all the awkward gaps
Last year 64% of the questions asked in our forum were answered within our DIY project pages at www.diydoctor.org.uk/projects.htm The project pages are now separated alphabetically and your answers are accompanied by diagrams and the ability to see, and buy, the tools and/or required to complete your project. Use our search box to look for your answer and save a great deal of time and money!