Oil in air filter housing?

[minisandfords]

Having changed spark plugs & fuel filter to try and cure a weird misfire (which I think has worked) I noticed a residue of oil inside my air filter housing. Car has done 135k (1.4 petrol) but has been looked after mostly so shouldn't be too worn I think?

I'm guessing this is coming from the crankcase breather hose? Would this affect emissions come MOT time?

If so what would happen if I left it disconnected, or blocked the pipe it at the air filter end?

Thanks in advance!