PierreCT

470's were pretty much standardised and IMHO one of the least variable of all of the calibres.

However, do this.

GREASE, LUBE WITH OIL etc the inside of the barrel.
Get a 45 - 50 cal SOFT LEAD Projectile or piece of LEAD Wire about 1.5cm long.
LUBE / OIL IT UP
Get a Hammer made of Wooden or Plastic
SLOWLY and GENTLY from the MUZZLE end tap it in until
the lead is in the bore.
Then get a piece of Wooden Downling - smaller than the bore size - and tap it is from the chamber end while the muzzle is rested on a piece of wood - the objective is to exand the lead to the full size of the bore.

NOTE - Make sure the Dowling Rod does not expand too much - if it does, saw off the expanded bit and start again - don't get the piece of wood stuck in the bore !!!

Once the lead is expanded, you can either tap it in from the muzzle end so it comes out the chamber or just push it out the muzzle (won't be as accurate but will give you a good idea).

Once out, get some calipres and measure from the EDGE of one of the raised pieces to the EDGE of one of the other raised pieces. REASON for this is English gun tend not to have exactly opposite grooves and lands.

Hope this helps.

PM me if more info needed.