Thanks for the replies. Just to clarify:

The front plate of the letterbox has 2 receptors which accept the
bolts. The thread inside one of these receptors has completely
disintegrated, taking part of its casing with it, so it can't be
rethreaded.

The reason why I suspect I may have to refill the bolt holes is
because they are *very* close to the rectangular hole and I fear that
enlarging the rectangular hole may cause it to 'merge' with the bolt
holes.

A tale of woe, I know! Perhaps I will get myself onto an evening
course in order to learn how to do these things properly, but I need
to sort this letterbox out ASAP - it's a shambles at the moment.

Thanks again for the responses.

Cheers,

Hamish