Excel 2007 : Creating a list of Hyperlinks

I'm trying to create a long list of hyperlinks to quickly check the credit status of a number of sales prospects.

Data takes the form:

Company Name, Registration Number
Company A, 00012345
Company B, 00023456
Company C, 01234567

For each row i want to create a hyperlink to:

"http://app.creditsafeuk.com/CSUKLive/WebPages/CompanySearch/CompanyIdentification.aspx?CompanyNumber=" & <Registration Number>

Thought I'd found the answer with

=HYPERLINK("http://app.creditsafeuk.com/CSUKLive/WebPages/CompanySearch/CompanyIdentification.aspx?CompanyNumber=" & B2, "Creditsafe-" & B1)

Seems to work for row 1, but when i copy the formula down another 1000 rows every row resolves to the same URL.

How do i force Excel to re-calculate the URL's for each row?