Some time ago I created a formula which works.

{=SUM((RegularJobs!M2:RegularJobs!M222="A")*(Regul arJobs!J2:RegularJobs!J222="3.3"))}

This provides a count of the number of rows where the M column has an 'A' in place and the J column has a '3.3' in place.

Works a treat. However I've just attempted to produce the same formula in another cell and it doesn't work - why? Because the system somehow added the curly brackets { to the formula line last time and I can’t recall how I did this.

Please for my sanity explain how to add the {

Thanks

It's an array formula. To get the braces you have to complete the formula entry with CTRL+SHIFT+ENTER instead of just ENTER.

This formula gives the same result, is a bit faster and does not have to be array entered:
Even better, if you are using Excel 2007 or later, you can use COUNTIFS().
If you're interested in reading more about array formulas, I wrote a detailed introduction tutorial here:
http://www.xtremevbtalk.com/showthread.php?t=296012

If you're interested in reading more about SUMPRODUCT(), Bob Phillip's page is a very good resource:
http://www.xldynamic.com/source/xld.SUMPRODUCT.html

Hope that helps....