I’ve got a 2007 rrs with 20” wheels, got a puncture today and put the spare on that’s a 19” space saver
My question is will a full sized 20” fit underneath as a spare

Yes, it should do if you're using standard size tyres. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

I've got a 21" wheel with a 275/45 tyre underneath mine, close, but it fits.

As a tip, I have a large carpet tile with a slit from one edge to the middle, that sits on top of the wheel to protect the wheel face rubbing against the metal of the car.

Dave 2010 3.6 TDV8 HSE (Gone)
2011 Discovery 4 Commercial SDV6 (Gone)
1980 OBLIC 4.0ltr Range Rover (went a long time ago)

Thanks Dave a tiles a very good idea

I have 20 inch (275 40 R 20) on mine and the full size spare fits just fine.

I like the idea of the carpet tile though. Robin
2008 Range Rover Sport TDV8
Gone:
Jaguar S-Type V8 4.2 Sport
MGZTT 190 2.5

A great idea, underfloor carpeting . The dealer who I bought my D3 from undersealed not only parts of the car but the spare wheel also, when he tried to deny it I said lets take the spare wheel down and sure enough clear where the tyre and the spokes of the alloy were.

Best mod you can do to your vehicle is fit a spare wheel protector. This stops the spare being stolen - something that is easy to do and results in a bill for a new wheel/tyre and a new sparer wheel winch too! 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed