OK, firstly this is not my bike but a friends.

It is a cheap X-Mart/Halfords bike. I cannot remember the number of gears, but I am going to say 24 just so I can identify the issue.

She told me that when she is riding, she gets a ticking noise when she selects 7th. On questioning her she told me she only uses the smallest 3 or 4 rear cogs with the large and middle chain rings, never with the smallest. So I could rule out cross chaining.

I turned the bike over, resting it on saddle and handlebars, popped it into large front and small rear(8th), everything seemed fine, moved the shifter to 7th and the dérailleur shifted, but not sufficiently to change gear, hence the clicking. Moving the shifter up to 6th caused the chain to engage 6th skipping 7th altogether. The rest of the gears seem fine.

I haven't taken it apart - but on a close inspection to my untrained eye it appeared as if the gap between all the gears up to 7 was narrower than the gap between 7 & 8. Could this be possible? Could the last gear have been put on back to front and then the limit screw adjusted to make it work - or am I seeing things and the problem lies not with the perceived gap between the gears but rather a calibration or other issue?