Replacing built in under-workstop oven with larger unit

I'm at my mum's for the weekend and her oven died on her yesterday, so we picked up one on ebay this morning.

The old unit was a single oven, built-in, under-worktop. The new one is also under-worktop, but a double oven.

I've removed the old oven which leaves the following hole:

By my measurements, the new oven will be flush with the plinth, so even if I could take the shelf out and...

Replacing built in under-workstop oven with larger unit