Oyster Max help with tandem!!!

Hi everyone, I was wondering if any of you could help me. 

I recently purchased a second hand oyster max (1 year old) for my 19month old son and will be for our second baby aswell when he/she arrives in December. 

i love it already, I've taken it out on a test run with just my son and it's a great pram so no fault with it - that was until I decided to practise attaching the car seat and when I tried to remove the black tandem plugs they just won't budge. I've been studying the instructions and I'm 99.9% sure I'm pulling at the correct bits etc but I've given myself a few scrapes on my fingers trying and am sure it's not supposed to be this hard! 

I purchased the tandem adaptors but it's the actual black blocks that come in the pram that are stuck, I assume they're there to hide the fact It's a tandem if you only have one child? 

Please if any one has had something similar happen, can you tell me what your solution was? I'm now panicking that ill have to go out and buy a second pram/ chassis?!!