Hi CK
CONGRATULATIONS.
Fabulous news 
Try not to feel too stressed as any sense of disorganisation/lack of communication. It seems to be par for the course even in very good adoption agencies.
You asked Should I get two of the butterfly albums so that we can do more pictures / recordings?
Your call, I would because I tend to go overboard on everything, but I think I would only introduce one at this stage, a later one with other family members prior to meeting them may be nice. If you want two and can afford it get them now, you will have other things to worry about later but I would agree with Jameme two are not necessary.
We also did a scrap book and included stickers and random big and small pictures of Peppa Pig and Thomas the Tank (as he liked them) - which is harder to include in an album - as well as photos.
Re Apart from photo's of me and my partner, should we add other pictures of our families (they don't live with us) and pets (2 cats)
Yes, you and you partner and cats, not other family members who do not live with you at this stage.
We had photos of us all as a family (birth dd, then 9), then individual photos and also our cat, garden, local park and house. We got 8 photos, so you could cut down on park/outside of house and garden and just go for their room, you two, the cat and one other thing or you both with cat, you each separately and then the bedroom etc. Your call! of course! 
We (me and dh voiced different pages) said stuff like 'I'm Italian, I'm your new mummy', and ended with 'We can't wait to meet you.'
Yes wonderpants etched on my memory still and little one (now 4) still listens to it.
Re Should we refers to ourselves as "mummy" and "daddy" in the voice recordings?
yes, we said new mummy and daddy but also gave our names.
Re What sort of things should we say in the recordings?
I said stuff like 'I like takewan-do' and my hubby said something like 'I like SCUBA divining'.
ReAny other creative / tried and tested ideas of things we could do?
DH bought a blanket with ds's name and date of birth on and I slept in bed with the blanket for a week or two before we passed it on to the social worker to give to ds (then 3).
Our dd directed a dvd and my hubby filmed it. Ds watched it a lot. It included our cat and lots of walking about, going to the park and eating sausages!
We also took a teddy and a book and a toy but that was because we wanted our dd to bring a gift. We spread the gifts out over 8 days of intros. For a baby I would limit it to one or two items only.
Re Sorry to ask so many questions, if we could have a decent conversation with our Social Worker (who is lovely but extremely scatty) I would have put this all to her. We been trying to stay chilled and just prepare for this huge new step in our lives, but I'm now getting quite stressed as I hate not being organised!
STOP stressing, your social worker might be full of ideas or not. But you have us. 
Also, really this is all prep but it is fun. Just think of all the options, make a list decide what you can do (photo book of 4) and cross out what is out of your control (photo book of 8) so make the photo book count by choosing the best photos and remember you may not like your voice but your new child will grow to love it.
If you want to do a blanket or snuggly get it in time for meeting and offer to post it off once you have slept with it for smell if not time to do before. Smell is more important for babies than older kids, I think. If you wear a distinctive perfume it is easier, I do not! I smell of soap and tea!!
We also took a teddy on the first day and said to ds, please look after this teddy until we see you tomorrow. We asked foster carer which teddy to buy.
So we did talking photobook, scrapbook, dvd and blanket before meeting; during intros we took teddy and a book and our dd bought a toy for ds too.