Harry Tuffin's Transport Caf� is open 7 days a week. It's about half way between Oldham and Tenby and only involves a very small detour.
Come off the A483, after passing Welshpool, and turn left onto the A490. Follow this for about 8 miles, to Church Stoke. When you reach the A489, you're only 300 yards from the caf�. (It's not clear from the website whether you turn left or right onto the A489 but it shouldn't be hard to find. You could always phone for directions):
http://www.transportcafe.co.uk/harrytuffins1.h tml
To return to your route, simply the follow the A489 towards Newtown and then pick up the A 483 again. (The detour adds just 8 miles to your total journey).
Chris