Fill Down Table In Access

Ok Ive looked and looked and read a multitude of threads to no avail. Please help me on this one.

I have imported a table into Access, I had no control over the original format of the table so Im stuck with the data as it is in that original doc.

So as it sits now in access..
index mnem client name
--------------------------------
1 310055 blah foo
2 blah2 john
3 blah4 joe
4 310099 grr12 bob
5 blah2 john

etc. * about 50,000 literally in size.

Looking at the empty cells in the 'number' column I need each of those to be filled. to end up with:
index mnem upin name
-------------------------------
1 310055 blah foo
2 310055 blah2 john
3 310055 blah4 joe
4 310099 grr12 bob
5 310099 blah2 john

I did this once in VB/excel but it was prohibitively slow. Im positive there is a better SQL/Access solution but just cant quite get it. My best effort to date is:
DLookUp("[mnem]","upin","[index]=" & [index]-1)
criteria mnem Is Null
In an update query updating mnem.

Downside all I get is say there is a block of 4 empty spaces in a row before the next mnem comes along, only 1 cell will be filled per run of the query.

i.e.
index mnem upin name
-------------------------------
1 310055 blah foo
2 310055 blah2 john
3 blah4 joe
4 310099 grr12 bob
5 310099 blah2 john


I figure #3 is somehow looking up at #2 before its updated and taking that empty value. ..just cant figure a way around it though.

I know this has been done before but no matter how much googling and forum diving Ive done I havent found something to specifically address this.

Thanks in advance for your help