Volvo V50 2010, drove fine but then noticed oil leak, popped the bonnet & oil had filled my coolant reservoir & caused it to split slightly on top where it was leaking out of. checked dip[stick & at a good level so drove home approx 6 miles fine. Its thick clean oil in reservoir, not a sluddgy mix, any ideas what could be the cause? thanks

 

possibly the oil cooler gasket?

 

If raw engine oil has floated to the top of your coolant bottle - that is usually caused by a bad oil cooler (if equipped) allowing oil into the cooling system - it's a mess to get clean (after replacing the oil cooler)

You are in GB - what engine is in your car?