For Iva can I have mixed tyre sizes per axle?

Ie on the front 195/55 r15
On the rear 195/50 r15

Reason being that the Speedo is reading wrong when I use the wheels that came with the donor which are fitted with tyres to large and I just so happen to have the 2 wheels with the correct size

I would say no. They check them that they are the correct rating etc anyway and they will pick up on it. However it will depend on your inspector most likely! Call into your local IVA station and ask is the best bet, or fix the speedo issue which you will need to anyway!

Why would it matter? I had 205 rears and 185 fronts for the IVA, which is quite normal.

As long as the tyres are the same on the same axle then no problem. It's not uncommon to have 2 sizes but unusual to have larger on the front... but there's no rule against it.

^^^^^ What Adi said!