Oxygen sensor and coolant temp sensor faults, help!

[Beaumotor]

Good afternoon,

I have a 2003, Petrol Berlingo multispace 1.4l. 80k miles. 

My the engine management light came on. Faults that came up were:

P0130 O2 sensor circuit malfunction bank 1 Sensor 1
P0135 O2 sensor heater circuit malfunction in bank 1 sensor 1

My exhaust does have a leak, towards the front before the catalytic converter where the two parts connect with bolts and springs, one bolt has come off (see attached photo of diagram). Apart from a slightly noisy exhaust there were no issues driving at that point. I have ordered replacement fixings for the exhaust and if I can't fix it myself will book in with mechanic after pay day. I wasn't sure if the faults could be due to the exhaust leak or if it's the actual sensor. I didn't investigate further than changing the fuse.

Cleared the fault and no issues for a few days.

Next issue the engine is pulsing/chugging at idle and juddering at low rpms. Once up to speed no issues. Engine management light back on.

Following day I'm driving along and suddenly the STOP light flashes and beeps, and temp gauge shoots up to the red. I pull over, read for fault codes and now have two faults, one is different...:

P0135 as above
P0115 Engine coolant temp sensor 1 circuit.

I reset the faults, checked coolant (it's fine and no signs of engine overheating) and as soon as I reset the faults the temperature gauge dropped right back down.

I continued driving home and it happened again but this time I couldn't stop so risked continuing driving. Within 1 minute then temperature gauge went from being in the red zone, and dropped immediately to 90c. Minutes later the same thing happened it went back up... 

I'm confident the car isn't actually overheating. Now the STOP warning doesn't come on when the engine is cold. The engine is still juddering at idle, but the STOP light comes on around 5-10 minutes into driving. It then drops back down within minutes most of the time. Theres no smoke from the exhaust however it does smell like the engine is running rich. The P0135 and P0115 continue to come up. .

My question is what could be the cause of this? I can't find anything to suggest the two faults are related, but I'm by no means a mechanic. I know I should have immediately got the exhaust fixed, and then got a mechanic with the first faults and usually would, but due to current circumstances (cancer diagnosis and off work) I've not been able to. It's obviously an older car and I don't want to chuck money at replacing oxygen sensor etc to find that wasn't the issue and the actual issue isn't financially viable to fix.

What can I do to diagnose the cause? Is there anyway this could all be down to the exhaust pipe leak? I won't be driving it until I can figure out the cause. Any help or suggestions would be very much appreciated thank you.