From my local tool shop I got a
3/8" drive adaptor Laser 7 piece Star Bit set containing T20, T25, T30, T40, T45 & T50 bits.
They didn't stock a 3/8" drive T55 bit so I ended up with a T55 in 1/2" drive made by Newsome. On the back of the packet it says it fits Cylinder head bolts on Ford B OHC Engines and V6 Engines August 1985 onwards - so maybe available from a Ford dealer ?