I currently have 205/50/R17 tyres fitted on my Focus Mk2 2006 TDCi but have an option to change to new alloys/tyres that are 215/55/R16. Using this tyre size calculator http://www.etyres.co.uk/tyre-size-calculator.htm it says that the diameter is only changing by 0.94% (6mm) which is well within their recommendation of +/-2.5%. My concern though is that 215/55/R16 is not given as a tyre size for my Ford Focus (on the tyre pressure sticker on driver's door, it has 205/55/R16; 205/50/R17 and 225/40/R18) so would it cause any problems or be inadvisable to make this change to 215/55/R16?

Any advice/thoughts much appreciated - I'm a newbie to this!

It's all about track and contact patch when it comes to tyres IMO. The diameter change will alter the rolling circumference of the tyre maing it either accelerate faster with a lower top end (smaller diameter) or slower acceleration and a greater top end (larger diameter). When it comes to the width of the tyres then it alters the track and contact patch ergo the handling characteristics. The change you are making is relatively small but would advise a tracking check once it is done. You will also notice a slighty difference in handling too as the wall of the tyre is increasing from what you have so you will get a little more wall flex from the slightly higher profile. The recommended sizes on the car would be to ensure that your speedo reads correctly. The change you want to make shouldn't cause any real problems realy down to how picky you want to be.

Another good wheel/tyre calculator, like the one you linked to is this one - http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

Cheers

Rob