Just bought a MRB. What clothing and armour (if any) would you recommend getting?

The Voodoo Bizango has a great rep, it's had many 5* reviews, good choice!

Any helmet is better than no helmet, so buy what you can afford. You do not need a full face helmet. When going to buy a helmet, fit is very important as difference brands use different lasts. Go to a bike shop with a good range and try some on - in fact go to several bike shops! Do not buy one if you think it makes you you look like a d1ck! If you do that then you won't wear it. Many XC style helmets are very much like road helmets, but they have a peak. So if you can't find what you want wear the one you have until you do find one! I now wear a trail helmet which has more side and back protection than an XC. I wear it because when I fall off I really need it, but they are more expensive.

You will need some gloves. Gloves are not there to protect your hands from the bars, they are their to protect your hands from the gravel when you fall off, and from the nettles and brambles as you pass by. So there is no need for thick padded palms, but don't get gloves with lacy or paper thin backs either.

Protective glasses. This is a personal choice, but I think an essential one for eye safety. If you don't wear specs normally or use contacts, riding specs can be really cheap, otherwise not. I wear them because they stop brambles and twigs doing damage (I had many close calls before I woke up). They also stop flies and mud from getting in there.

When I first started with MTB there was a phrase in common use; "all the gear and no idea". It was aimed at the rider who turned up with £5k of bike and kit and yet held everybody up on the trail because they had no idea how to ride. It is unfair because everybody has to learn, and why are they expected to start on crap bikes and with manky kit? It has the smack of jealousy to me. However, I was surprised at how much that phrase affected me, but in a reverse way! Because I was a beginner I didn't believe that I had "earned the right" to wear knee and elbow guards (yes, I know, pathetic). I foolishly thought that wearing guards shouted that I am such a good rider that I need this stuff, because I ride hard stuff, Yey!! Yet I plainly was not a good rider - a perfect case of all the gear and no idea! But because I was learning, I fell off a lot and NEEDED the guards more than most. Even though I wasn't going fast, just faster then my skill level permitted, I picked up a lot of scars on my knees and shins. My shoulders still suffer from injuries picked up years ago. So I now wear elbow guards and knee & shin guards every time out, every time! Don't go straight out and buy strap-on hard faced guards before you've ridden anywhere. Instead, keep an eye out on what other riders use. Ask them what they use them for, what they like about them etc. I wear lightweight pull on types by Troy Lee Designs, from the Shock Doctor range. They are perfect for the riding I do as I am not a downhill warrior that needs protection from high speed impact with pointy rocks. What I need is protection from my pedals, nettles, brambles, barbed wire and from impact at the speed I tend fall off at (0-20 mph). Impact with trees, walls, my bike, the ground. They are washable and breathable and I wear them all year round. My shorts are long enough to go over the knee guards even when pedalling. My elbow guards are slim enough to fit under my jacket.

If you care about what people think, then don't wear skin tight lycra. Otherwise go for it! If you are a style victim, go for something a bit looser. Padded undershorts are a must, and you existing kit is fine in that respect. Use them as underwear but not outerwear! Decathlon make good budget kit and Aldi and Lidl also have stuff in their shops from time to time.

One last thing. I know that you are an experienced road rider, but MTB requires some extra skills and you need time to learn them. Either go on a skills course, or ride with someone who is an experienced MTB rider. Take your time, be safe and enjoy!

You're going to use SPD shoes and cleats? I've got tosay that wasn't my choice when I went from road to mtb.... flats for me. I want to able to bail out quickly if I need to.

If you are used to riding with clipless pedals on your road bike you won't have any trouble with SPDs.
Buy a pair of the multi-release cleats (marked with a letter 'M' - SM-SH56) to make bailing a bit easier and away you go.