See “what bike rack” thread. You’ll see that gazthediver bought a Thule Velospace 2 and the tailgate won’t open! As I posted on that thread I have the official Mercedes towbar mounted 3-bike carrier which will allow the tailgate to open (just) when it’s dropped. The Mercedes bike carriers are actually made by Atera and the 3 bike model is virtually identical to the Atera Strada Evo 3 (Apart from one of the arms). It’s a high quality carrier and is readily available in the UK. It folds and fits into a canvas bag which will fit in the rear of the MP if you want to put it away. I find this unnecessary as it locks onto the towbar. BTW, the canvas bag was an optional extra with the Atera, but included with the Mercedes model.
Also, note what I posted about the dealer who fitted my Westfalia removable towbar - he’s also a Thule agent and doubted ANY Thule bike carrier would allow the tailgate to open. I think its something to do with the length of the tailgate preventing this.
I've had a look at the German Mercedes website where the 3-bike carrier is currently availalable for €559.90 (about £500 at current per rates) - reduced from €669. You only pay €7 carriage from Germany, but you can return FOC if you don’t want it! But you have to pay by international money transfer which cost me an additional £15.
Roofbox.co.uk also have them reduced from rrp of £647.50 to £479.95 - carriage is £8.50. Looks a pretty good buy, but storage bag appears to be no longer available and I seem to remember it was an extra £50 or so - expensive, but very thick canvas and high quality. So it really depends if you want a bag.
Delivery from Germany took about 3 to 4 days I seem to remember.
I'm very happy with my Mercedes/Atera carrier. Hope the above helps.