substitution within a vim function using regex - function

Question

I'm trying to turn a macro into a Function. Whilst I can save the macro I'd like to make it a function I can call.
I'm de-duping a specific element in a large concatenation of xml files using the following
G$b"myw0"n4ywdd:264501?^Rn/\dcw^Rm^[
Which gets parts of the last line into registers m and n then goes to a known line above the dedupe data and searches backwards for the contents of register n and replaces the number in the line with register m. The known line is to make sure I find the first occurrence that's not part of the de-dedupe set. Works fine if a bit slow when running 600 times.
On the function I've managed to duplicate everything except the cw with the contents of register m part.
function! MakeUnique()
let i = 0
while i <= 1
:200
:let #n=""
:let #m=""
:normal! G
:normal! $
:normal! b
:normal! "myw
:normal! 0
:normal! "n4yw
:200
execute "call search('".#n."','b')"
let i += 1
endwhile
endfunction
I've tried a few combinations of substitutions along the lines of the following but with no success.
Try 1
Register n = <dataid>389 and register m = <dataid>333
I changed the Line at the end of the file for this try to and changed "nyw to ny$
<dataId>333<dataid> <dataId>389
I would expect that subcommand when executed would read . s/<dataId>333/<dataId>389/
let subCommand=". s/".#m"/'".#n."'/"
exec subCommand
But given the result it doesn't seem as if #n is expanding
Result:
</dataid>
Try 2
In this case #n = 389
let subCommand=". s/[0-9][0-9][0-9]/".#n."/"
exec subCommand
This one works but the \d+ part may b 3 - 5 digits and I can't get a regex to work in this case, ie
s/\(\d\+\)/".#m."/
doesn't work and I get a E486: Pattern not found d+
Questions:
Is there any way to use regex match in the let subcommand = bit eg let subCommand = ". s/\d+/".#n"/"
Alternatively how do I replace using something like
let subCommand=". s/".#m"/'".#n."'/"
Or is there a vimscript equivalent of cw^Rm ?
Data example, cut down to fit.
The first iteration of the macro will change 6415 to 8790 in the identified line below.
I'm trying to replicate that in a Function.
1 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
2 <createsaleRequest>
3 <sale>
4 <saleNumber>9987166</saleNumber>
5 <dataId>6315</dataId>
6 </sale>
7 ...
8 </createsaleRequest>
9
10
11 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
12 <createsaleRequest>
13 <sale>
14 <saleNumber>9988166</saleNumber>
15 <dataId>6415</dataId>
16 </sale>
17 ...
18 </createsaleRequest>
19
20 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
21 <createsaleRequest>
22 <sale>
23 <saleNumber>9987921</saleNumber>
24 <dataId>6315</dataId>
25 </sale>
26 ...
27 </createsaleRequest>
28
29
30 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
31 <createsaleRequest>
32 <sale>
33 <saleNumber>9987411</saleNumber>
34 <dataId>6415</dataId> <!-- Change this on first run through -->
35 </sale>
36 ...
37 </createsaleRequest>
38
39 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
40 <createsaleRequest>
41 <sale>
42 <saleNumber>9987411</saleNumber>
43 <dataId>6193</dataId>
44 </sale>
45 ...
46 </createsaleRequest>
47
48
49
50
51 <dataId>6315</dataId> 6789
52 <dataId>6415</dataId> 8790
EDIT
Added a cut down data example to try and make the question make sense.
Edit
Found the problem. I'd left the second . out of the match portion. This let subCommand=". s/".#m."/".#n."/"works and is significantly faster than the macro. Don't know how to close the question.
I would like to know what ". ." means. I'm guessing it's an escape pattern that works in vimscript.