Engine Management (EOBD) light has come on.

Have had diagnostic check done and its giving error code 209716H/2135830, does anyone know what this code refers to as the issue. Monza exhaust system back box changed last week, could this be a factor?

I think this 209716H is actually P2097-16.

P2097 is a post cat O2 sensor fault, it's recording a rich reading after the cat.
Rich is generally cool combustion so the O2 sensor produces a low voltage.

O2's aren't your normal sensor, they self generate an electric signal depending on the heat over them, anywhere from 0.1v to 1v.
Lean = hot combustion = higher volts.
Rich = cool combustion = lower volts.

There are generally two sensors on a engine with just one bank of cylinders (and 4 with two banks or V engines).

One pre or before the cat, this one regulates the fuel, it's readings alter how much fuel in injected.
It'll switch back and forth from lean to rich (0.1 to 0.9 or 1v) trying to balance everything out, Low v = rich so it takes fuel away. High v = lean so it adds more fuel.
With everything running ok you tend to get a level trim over time (around 0.45v average between the switching volts) which indictes near perfect fueling

One post of after the cat, this tends to monitor what comes out of the cat and doesn't tend to influence the engines fueling (though on some cars in some markets it can cause limp modes to stop the engine over polluting)
The ECU is just looking if the Cat is doing it's job and the signal is within tolerance (and not mirroring the pre cat signal)

The 16H is a sub section of the fault which helps identify the actual fault, I've no idea what this is sorry, I've never had any diagnostic kit that dug that deep myself.

As you've had the exhaust replaced it might be it's leaking, this will allow exhaust gas to escape and/or draw in air from outside.

This can screw the O2 sensors up, the extra air/escaping gas cools the pre cat O2, it fools the ECU into thinking the engine is rich so it tries to lean it out.
Or if the leak if further back in the exhaust system, it can just cool the post cat O2 causing it to think whatever is coming out of the cat is rich.

Many thanks for your reply.

Ive emailed the garage that done the work and hopefully they can dig down on the code and it is the exhaust leaking

It could be just the exhaust.
Quite often these sporty exhaust play havoc at MOT time as they're a bit too airy for the tester Lambda tests, they often have to stick a rag up the pipe and block some gas from escaping.
Your replacement might be having the same effect of the post 02 sensor.

I believe there's a bit of a difference between the earlier Monza' and the later ones.

When I bought mine (listed at £1000!) there were at least three different Monzas on the market.
A mild steel one painted black with chamfered tailpipes.
A stainless steel one (which I think it isn't as stainless as it could be), again with chamfered tailpipes.
Another with straight cut rolled edged chrome tipped tailpipes.

And now I believe there's a new one where the flap is operated by a vacuum servo rather than just gas pressure.

They sound ****** awesome though!
If you ever want a standard back box, I have one in the shed as some bugger stole my car (and Monza).