T-SQL Problem...selecting TOP 1 Of Each GROUP In GROUP BY?

Hi,

I was hoping someone may be able to help me with a tricky T-SQL
problem.

I need to come up with a SELECT statement that does basically the
following:

Select RCRD_REFNO, MAX(MODIF_DTTM) as MODIF_DTTM from
[StageDb].[dbo].tblPersonInfo group by RCRD_REFNO

However, I need to select ONLY the TOP 1 of each group (i.e. only 1
record for each unique RCRD_REFNO). The problem is of course that if I
add 'top 1' after select, it only brings back 1 record full stop,
rather than 1 for each group!

Now, I have previously come up with a similar query that DOES do this
successfully, but it relies on a criteria (such as a unique identifier)
-unfortunately, the nature of the table I'm using for this current
job means that it actually doesn't have a primary key, as it's
simply a staging area for raw data, and can even have completely
identical records in it. I think the only way I'm going to be able to
do it is to literally use the 'TOP' command somehow, but am not
sure how to adapt the above to implement it...I'd be very grateful
for any advice.

Many thanks