Solenoid, personally I use ebay. Seems to reap better prices. First job though is to get underneath and make REALLY sure that small wire (usually blue) has a good connection on starter. Without that it won't trigger solenoid - hence no start. If it seems slightly loose or badly corroded - replace the spade. Poor connections are a common cause of intermittent starter problems and its worth paying attention to starter before the snow/ice arrives. Theres also a thick braided wire that goes into side of starter. It can corrode (inside or outside the starter) and eventually fail. Check it carefully and that it feels secure at both ends. All is fixable and it might pay you to remove starter after disconnecting battery and giving the connections a really good clean with a wire brush. You could also check state of carbon brushes (about £7 to replace) by popping end cap off:
https://www.youtube.com/watch?v=gNKSeAAd3rk" onclick="window.open(this.href);return false;
You don't need any special skills - just a determination not to be fleeced for a new starter. DO NOT test it off the vehicle though as one end relies on a bearing inside bellhousing, and running it 'free' can damage the internals.
I can't add much more but if you need specifics of testing, theres lots on here already regarding ignition switch etc.
I have two starters. One is on the vehicle and the other is already overhauled and ready to fit should the one on vehicle play up. After 30 years of the same faults everyone else experiences, I found that is the most frugal and sensible way to go

So may people exchange starters that could be easily fixed at home.
I keep meaning to do a full starter rebuild as its straightfoward and noone seems to have done a full video with all the potential faults and how to fix them. Sure there will be those that just part with £150+ regardless but thats up to them
