Hello David, thank you for your reply and apologies for not replying sooner.
Like you the series 3 & 4 from my childhood is not too bad and it is the series 4 that I have bought for extending that, although I have tried to be selective, tends to corrode a little more.
The layout is in an insulated but unheated room inside the house. The corrosion is only very minor, not really problematic if I run trains regularly but I want to prevent further degradation. The problem is more prevalent on the steel series 6 points that I use with modern code 100 on a separate part of the layout. These points are used because they will enable running of older and modern stock. I did not try the Deoxit D5 on grounds of cost but after a conversation with John Bristol of Deluxe materials went for Track Majic. I have used on half the layout so far and am not confident about results I am seeing. I have sought advice from Mr Bristow and am awaiting his reply, so?