Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
My first reaction would be to draw HMRC's attention to the P60s as they would appear to have all the data available already. That Department is notorious for asking for information already in their domain. It is always my policy not to assist in official laziness!
For your own information, as Swiss taxation tends to be much higher than the UK equivalent, then under the Double Taxation Treaty between the UK and Switzerland any tax paid there is allowable as a tax credit against any UK taxation on the same profit stream. Thus any further UK liability is unlikely. As usual HMRC are barking up the wrong tree and going after individuals instead of the big boy tax evaders or those with aggressive avoidance schemes and expensive and argumentative accountants.
I do hope that my reply has been of some assistance.
Send copies by all means, but not the originals. They are almost impossible to replace. This separate unit of HMRC are notorious for getting hold of the wrong end of the stick if they can even find the stick at all! If your tax affairs do not need updating why the inquiry one wonders?
Thank you for your support.