Unusual Oil Leak on a Fiat Brava 80 SX 1.2L on a "Y" plate. ?

My Fiat Brava 1.2L 80 SX leaks oil from the top of the dipstick hole. Pressure appears to build up inside the engine which pushes the dipstick up and sometime completely out of the hole and then the oil leaks out. It runs fine apart from this! It is an old car that I've inherited and it is somewhat overdue for a service but wanted to run this problem by someone before committing to possible great expense at the garage.
Any ideas? Would the simple act of changing the oil and filters sort this?
Many Thanks.