Thought I heard a slight noise when going up the drive, now exhaust is making an awful noise from the front, seems to be somewhere under the engine itself.

Suppose I'll have to ring the garage in the morning, any ideas what i've done, cost?

it could possibly just be old age and it's just a hole.

One of your exhaust hangers could have broken, a bolt or clamp could have given way, or as you say the exhaust itself may have blown. Not to worry you (but hopefully prepare for worst case scenario) I would hope it's not the catalytic converter which is the most expensive part of the system.

If you can, lay on the ground with the exhaust system cold. Try lifting bits up gently (well as much force as needed to lift) to check for play in the joints. Check the exhaust hangers are intact. Look for any obvious holes. Then start the engine and preferably have someone hold a rag at the exhaust exit and put your hand around the system to check for any leaks.