you guys overtake on the left, correct?
if the car that you overtake hits you, it's the general concensus that you were the the one who was trying to get into a gap that wasn't big enough.
the other driver shouldn't have to slow down to let you in.
how fast he is going is (almost) irelevent as you should've been able to judge from your mirrors whether or not you would be able to merge in safely.
if you got hit, that would mean that you were not able to judge; hence your fault.
in this case, you were also in the lane that was merging into the other. seeing as this is usually sign-posted way before the end of lane, you also would have plenty of time make safe judgements.