I have a drop down list where a user can select the month (cell I6).
I also have a data set on another sheet like this:

jul;aug;sep;oct;nov...Total;YTD Total
100;250;350;420;530...Total;YTD Total

Assume date range starts in A1 and figures start in A2. Also, the fiscal
year starts in July, not January.

What I'd like to have happen is that depending on the month selected
from the list - for that selection to drive a YTD sum
calculation/function, i.e. If the user selects October from dropdown
list - then the YTD total sum will only include months upto and
including October only (i.e. July, Aug, Sep, Oct).

Try this for the YTD total:
=SUM(OFFSET($A$2,0,0,1,MATCH(I6,A1:L1,0)))
This assumes that the same month name spellings are used in I6 and A1:L1.

Modify as needed.