An Update:
I tried reinstalling Word, but this had no effect. I then located where I think the dictionary files should be located:
C:\Users\david\AppData\Roaming\Microsoft\Spelling\ en-GB
There are three files there:
default.acl - 6 bytes
default.dic - 240 bytes
default.exc - 6 bytes
I opened default.dic and this contains just 13 words that I have added, but this is just a fraction of what I have entered over the years.
I really could do with some help as Word is currently ignoring any spelling errors (the wavy red line) so it is hard to spot any typos, etc.
Thanks for reading,
David