The lambdas 'above cat' are under the bonnet - you'll see them screwed into the exhaust manifold down pipes if you look between the engine and radiator. They'll have cables coming up to near the air on compressor on the left as you look at it and just below the engine cover on the right. Lambda 2 is nearest the battery if I remember correctly.
I would check the connectors first, they have a (quite stubborn) push lock but you should be able to just disconnect them and make sure the connectors are clean and undamaged. Bent pins will stop the preheater working properly. Use contact cleaner rather than WD40.
The 'below cat' lambda is in the main part of the exhaust, maybe after the cat itself.
The codes you have are 'stored' rather than 'fatal', so may be intermittent. You could try using ecu scan to clear the codes and see if it runs any better or if they return.
If it's not a dodgy connector they made need replacing - I did both precat ('above') lambdas a bit ago. Unplug the connector and unscrew the sensor; they'll have been in a while so might be a bit stubborn. 22mm open spanner or mole grips, WD40 or similar if they're stuck. Screw in the replacement by hand, nip up with the spanner. Getting the old ones out is likely to be the hardest part.
Same for the 'below' sensor except that it's under the car, so less accessible and more likely to be corroded in place.

 

Couldn't of asked for a better response.
Took it upon myself to check the connections and all was ok so I went ahead and purchased 2 new sensors.

I've made 2 attempts to remove them to no avail. You're right about it being the hardest part.
Appreciate the response Iron Wizard!:thumbup:

 

Glad to be of help!

You can get specialist lambda sockets, which might give you more leverage, but I couldn't find one at the time. The other way to get them out is to cut the cable and put a 22mm socket over the sensor - but that doesn't help if it turns out they're not at fault.
You could try warming the manifold by running the engine for a bit so that the part they're screwed into expands. Might help, might not!

 

Try a good quality penetrating fluid regularly applied for a few days .. Not WD40 ... PlusGas or similar.

The lambda sockets are good but I had to clamp mine to the lambda hex with a jubilee/mikalor clamp as it started to open up.