Wow, great answers guys.
And yes it is the same Sabre, and it is fantastic which has inspired me to push on with the 5.
I want to be able to change between 4's and 5's without worrying, I can pick up a 5 now and play it, but I am sort of treating it as a 4 with an extra string instead of the whole entity approach, which I know is wrong. Also I realise you can't just drop these lower 'bomb' notes in as they do tend to be quite noticable, so I must stay in context of the song of course.
I agree, it is probably one of those, practise is everything scenarios, just as when I was learning backing vocals and playing bass together, thought I'd never crack that, now it is second nature.
Thanks all and keep it coming.
Cheers and Merry Christmas
Stuart