Why did you want to use a 9spd chain on a 7spd setup? If everything other than the crank is 7spd, even the cassette, just use a 7/8spd chain and be happy. 7spd cassette+ 9spd chain is not a good match. Except you have 9spd chain already on the shelf, but don't have the 20 quid to buy a new chain. On the other hand, 7/9spd chain on 9spd crank is no probelm at all, because the "inner" diameter of both 9 and 7spd chains is the same. Only the outer diameter is different, hence the problems that araise using 9spd chain on 7spd setups.