Hi Mike,
How did you spool your Hardy line? If you pull line off the side of a spool,it puts a twist into, you may know this already of course! When spooling any new line, stick a pencil through the centre of the spool and wind it straight on to the reel, either stick the pencil between your knees or have someone hold it so that the line is coming off the bottom of the spool.
The SLX #7 would be my choice, slightly heavier than a standard #7 line, colour change to keep things repeatable and just generally easy to cast at most fishing ranges without having to put too much line in the air. The £5 change from your £40 will get you a few tapered leaders.
Cheers
Ben