Is the VAT number something where you need the same exact value to appear on every invoice or is it a different value for each customer or invoice?
Any changes made in layout mode change the design of your layout. If you use the text tool to add text to this layout, what you add will appear identically on each record.
If this is data that is not the same for every invoice, then you should define a field in your database and then enter that data into the field while in browse mode.
No the VAT number wil be different for each customer. There is already a field for the VAT number to be added but when Im in browse mode I don't know how to actually edit it. I've tried clicking on the various layouts but nothing happens. No box appears for me to click into and change the text?
Is the field present on the layout? If not, you need to switch to layout mode and then drag an drop from the field tool in the ToolBar to add a new field object to the layout. Doing so causes the specify field dialog to open where you can select the VAT field to add it to the layout.
Note: The field needs to be added to the correct table in your database and selected from the correct Tutorial: What are Table Occurrences? or you can have problems displaying and editing data in the field. I can't tell you what the correct table or table occurrence is, as I have no idea how your database is designed.
Yes it's present in layout mode.
I've managed to add it and it's showing in summary orginal mode but when I flick to summary print it dissepears. Also something that was below it has now shrunk to tiny type?
"flick" or "click"?
This sounds like you have changed layouts--a frequent occurrence when you click or tap a button. The other layout may be designed to display data differently and may not have that field on the layout.