Caveot Emptor,Napier whilst being an EXCELLANT product, I believe, is NOT a traditional oil finish.What it is, is a modified finish that dries very quickly and gives as standard a very high gloss finish.The kit that Dig mentions contains most of the items required to do a good refinish.
Remember, as I have said many times before on this subject.It is very hard to bottle 100 years of TLC, sweat and care for your piece of wood, which is exactly what as been lavished on the Purdey you are trying to copy.
Alkanet (anchusa officinalis, anchusa tincturia) a European weed. Take about 70grms of beaten / crushed root, immerse in 100ml of pure turpentine for about 4 weeks in a sealed jar then add 100ml of linseed oil and reseal leave for another 3 / 4 weeks and then use as required.
You could also use a potassium permanganate solution of water which will soak much deeper into the wood (and of course raise the grain. AGAIN )this will impart a nice colour which you do have some control over, but remember the addition of successive oil coats will darken the wood progressively so do beware.
Have a snigger at my expense over the following anecdote.
About 10 years ago I had to refinish 10 of No4 Enfield rifles, I lavished about 3 coats of colour to the beautiful smooth wood and then numerous coats of linseed end result = fantastic dark mahogany coloured woodwork FAR too dark to conform with MILSPEC, a BIG NO NO. Result = A very very happy Colonel who's 'Boys' had better rifles than the rest, my boss received a letter of commendation and I got all the Officers shotguns to refurbish and I got paid to do it fulltime rather than as 'just a little favour'
I had plenty of driven days for about 3 years until the Colonel got a posting.