Thanks so much for your fast response and the effort you put in to help resolve this for me.
I tried it and it works!
The only small problem I had to overcome was as follows:
Line 11 of the code you suggested to put into functions.php is:
preg_match('/iframe src=[\'”]([^”\']+)[“\']/', $str, $match);
The small problem here is that some of my posts had valid code between 'iframe' and 'src'
For example: iframe height=”600” src=
Posts with this format were not being indexed so I changed them all to have nothing between iframe and src.
Could line 11 be somehow changed to cater for this?
(Maybe all it needs to recognise is “src=”?)
Anyway, thanks again.