The first problem is that anyone reading your post will have only a partial understanding of your business procedures. For example, when an item is "dispatched", is it ever returned?
If the items leave inventory and never return, you may want to use this method: Managing Inventory using a Transactions Ledger
If you are lending and returning items, you may be able to adapt the Asset Starter solution that comes with your copy of FileMaker.
Then you have the issue of sending out field engineers. This suggests that you track what items are in the possession of the Field Engineer and which have been distributed to clients. And your Engineer might have an iOS device such as an iPad or iPhone to record transactions. That device can "synch" back to the main database, or if you can count on a good wireless connection, link to the database as a client of that database. Lap tops and tablets that run the Windows OS are also not out of the question as a way to manage this.
Sending an email back to the office to document the transaction may actually be harder to automate than the alternatives that I've just described.
I was thinking of sending e-mail back with a csv generated file that maybe is imported back on the main database, if possible.
Yes, field engineers will have stock to install and stock may need to be returned - always a chance a unit fails!
Will look at the suggestions and sync back - good way to test installed ADSL unit definitely works
Other comments welcome