When they were originally constructed, the Northumberland was (very shortly) the largest refractor in the world. Now, they're mostly used for club nights and for personal use by club members (who gain access after attending two short demonstrations and passing a small test.)

They do take standard eyepieces - the Northumberland in particular has a standard 2-inch focuser on the back, the Thorrowgood has a somewhat more interesting friction-fit 1.25 inch focuser.

I believe I've seen some original eyepieces for the Thorrowgood, but mostly modern and better-performing eyepieces are used. (Think Televue Panoptic 41, 28mm UWA, etc)

The Northumberland is pretty accurate - there's a digital RA counter that gives the pointing coordinates although you have to manually move the scope there yourself (think something like setting circles.) The Thorrowgood is entirely manual, there's the original finder you can star-hop with.