You can't obtain the VAT number from anywhere except the company concerned. They are obliged (if VAT registered) to state it on any invoices they issue.
HMRC will not give out that information, and it won't be quoted on any forms filed at Companies House, so the link quoted above won't give you the info you need. The link does, however, show that the company hadn't traded in the year to April 2007 (as they filed dormant company accounts).
Are you sure they are VAT registered and have commenced trading?