Yay for the scan Jolly!
I'm going to give reuseable nappies a go too - will be buying a few different types to try but has it already been mentioned yet that Lollipop are doing packs for half-price online at the moment? It's a good bargain, and a friend of mine says they're fab.
I own up by the way - it's me who's the HR person on this thread - or at least one of them. My confession to make is I HATE statutory maternity rules whch appear to have been specifically designed to send pregnant women into a frenzy of head-scratching! Occasionally staff ask me to explain them and I used to go into an inner panic, grabbing the book to keep me sort-of-straight 
. I'm (slightly) better at them these days.
So yes, your MATB1 should technically be in by 15 weeks before the start of your expected week of confinement (which always starts on with Sunday rather than Monday, just to confuse...) so work that out if you like! Essentially get it in before 25 weeks and you'll be fine. In truth, most employers won't be bothered by a few days delay though so if anyone isn't going to see their midwife to get it by that point, just let your HR dept know there's a slight delay and 95% of them will be fine with that. If they're a bit more awkward and insist on getting it precisely on time, then you'll need to ask your midwife to post it or something.
Whilst I have my HR hat on, can I mention about risk assesments? They're a legal duty for employers under health & safety legislation, so if anyone hasn't had theirs yet, best ask when it will be - particularly if you're having any health/tiredness/mobility problems, etc. A good pregnancy risk assesment should identify if and what adjustments can be made (by you or your employer) to your job and/or environment to help make it manageable.