I use these:
Kelvin Service Station. Kelvin Road, WALLASEY, CH44 7DW
They nearly always turn up corrosion on the sills, but they're really fair, and have never charged me a retest fee.
I used to use Orchard MOT centre on St Paul's Road in New Ferry, they've got new people in though and it's not as good.
"C20 LET bang"