I have a form (Yr3 Update) which draws
data by query from two related tables.
Your statement above is the first issue that you need to look at. To be able to actually edit or update records in either of these tables when linked together in the query and then have the data from the query bound to a form will depend on what the relationship is between these two tables. If these two tables have a one-to-one relationship then it might be possible that you could be able to have the results of that query bound to a form and still be able to edit the data. The key word here is "possible".
However, if these tables have a one-to-many relationship, it is highly unlikely that you will be able to update records in either table in the query or in the form bound to the query. It is one thing to display the results of a query that has multiple table joined together and quite another to be able to edit the data in either of the underlying tables.
You can test this by opening your query in datasheet view and see if you can edit the data in the query in datasheet view. If you can edit it then you do not have a problem. If you cannot edit the data then you will need to take a different approach to updating your data.
As a note: I supposed that it would be possible to do what you are wanting to do with what you currently have but it would require the use of VBA code.
One possibility, which would also be a "no VBA" approach, would be to use a form/sub-form approach. This is where you have your main form bound to the first table and the second form bound to table #2. Then you insert form #2 into form #1 making form #2 a sub-form to form #1. Then you use the "Link Master Fields" and "Link Child Fields" to link the sub-form (form #2) to the master form (form #1)
Using this approach you will be able to edit the data in both tables. You then could use the Dlookup function in the "Control Source" of a control on the sub-form to populate one or more fields in in table #2 which is actually bound to table #2.
I hope this is not to confusing but take a look at and search for more information about the user of Master and Child forms or Main and sub forms.
Hope this helps.