The message
from "DavidKendall" contains these words:

Does anyone know, please, when I might find a suitable product or can off
any advice ?

I fitted bolts at each corner into the side timbers. If there's a gap
build it out a bit. Fit metal plates behind the bolts so it's a sod to
get a saw in and ideally bolt through plates at the front to stop
drilling the wood out.

Failing that...
http://www.saundersonsecurity.co.uk/...oor_Locks.html

http://www.saundersonsecurity.co.uk/...Door_Lock.html
(click on the pic to see how it works)

http://www.accessunderwriting.co.uk/...view.php?II=64

You get the idea - google!
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Skipweasel
Pay no attention to that man behind the curtain.