Different wheels front to back...

[Deluks]

I have 2 incomplete sets of wheels due to punctures, I have 3 of one size and 3 of another. Hence I have 155's on the front and 145's on the back.

I am correct in thinking that this is legal and will not class as a fail come MOT time?

 

[timtheenchanter]

as long as the larger are on the back and smaller front, its fine.

you can mix front/back but not side/side

 

[Deluks]

By larger I assume you mean wheel rim size?
I have the same wheel sizes, but the wider tyres are at the front.

 

[Nitronick]

Hence I have 155's on the front and 145's on the back.

You need to swap them round. Stick the 155's on the rear and 145's on the front and then your legal. Dont forget your vehicle speedo will read incorrect if you have the wrong size tyres on the driven wheels.