HI @SteCoxy
If you are creating an item in a list every time a form submission is made, you need to get the last id and increment by 1, before prepending your fixed string.
In my article I use get item to get the last known ID as you need to know the last unique id before creating the next one. Some folk use a separate list, others use the list item ID but you need to use something.

I also have two columns to keep track of the running ID and the prepended ID but you could use replace() to remove your prefix to allow you to increment on the integer, it's just a bit more work.

Above, assuming the column is called InvoiceID, you can add 1 on to the last number.
add(int(first(outputs(‘Get_items’)?[‘body/value’])?[‘InvoiceId’]),1)
You then need to pad with 0's:
concat(substring(‘00000’, 1, sub(5, length(string(outputs(‘InvoiceID’))))),outputs(‘InvoiceID’))
Then when you create your item, you can add
D&I-
alongside the output (instead of "Invoice" in the Title) from the above expression in compose.

If this doesn't make sense, please explain where you get stuck.
Damien