We are in our late 50s and have set aside 3 nights Sept 10-12 to camp in Yosemite as part of our 17 day holiday from the UK. We're driving from San Francisco. Without knowing the area any advice on where to reserve a site will be very helpful, given how large Yosemite is, as would be any tips on how best to reserve a site.
Although our preference is to camp, any advice on well located lodges, reasonably priced would be very much appreciated.

Here is the link to reserving campsites in Yosemite:

https://www.recreation.gov/recreatio...encyCode=70904

This link has valuable information on how far in advance you need to reserve, etc.:

https://www.nps.gov/yose/planyourvisit/camping.htm

Important note:

"Reservations are required all year for Yosemite Valley's car campgrounds and summer through fall for Hodgdon Meadow, Crane Flat, Wawona, and half of Tuolumne Meadows. Campground reservations are available in blocks of one month at a time, up to five months in advance, on the 15th of each month at 7 am Pacific time. Be aware that nearly all reservations for the months of May through September and for some other weekends are filled the first day they become available, usually within seconds or minutes after 7 am! For your best chance of getting a reservation, be sure your clock is set accurately and start the first few steps of the reservation process at www.recreation.gov before 7 am Pacific time. You can find more help making reservations and other frequently asked questions."

I assume you will want to camp in Yosemite Valley, which is not all that large.

But are you sure you want to camp? There are tent cabins in the valley so you don't need to schlepp camping gear.

You may have to camp outside Yosemite.

Look at this: http://www.exploreserac.com/10-littl...national-park/

and this:

https://www.fs.usda.gov/activity/sie...45420&actid=29

Sunday afternoon or Monday might be the best time to find a camp site, as many people leave after a weekend.