OK, the story so far.
I had all sorts of problems accessing WordPress, but this was solved by you kind folks here on the installation pages and WordPress has been running fine for the last couple of days.
However, today, for no apparent reason, it's stopped dead, with the message in the title.
Everything I've researched so far, suggests there is no index.php file; but there is.
If I place a plain index.htm file in the blog folder, I can access the file just fine, so clearly,
a) my browser is looking in the right palce, and;
b) it goes deeper than a missing index.php.
Whether I try and access the blog or the login page, I get the same "You don't have permission" message.
The only changes that have been made are as follows;
Last night, I stopped the DNS server, as I thought I didn't need it running. This has had no effect on my Apache2 server, with my regular web pages being served without errors. I've switched it back on this morning, just in case it affected WordPress, but it's made no difference to the result.
Reading the installation page, one common error noted, was a missing ?> end tag to the config.php file. I added this, but again, I've still got this problem.
I've also added a theme, but don't know how to restore back to the default, without access to the WordPress control panel. However, it should be noted, that theme has been running for the past two days also, without any problems.
I've checked as best I can phpMyAdmin for database errors, but I'm no expert. However, as far as I can see, it all seems to be up and running.
Once again, any help would be most appreciated.