Vertical stringing of your shots is usually indicative of a BEDDING issue: the wood-to-metal FIT of things. In the SMLE rifle (or the early Number 4 series) this is usually traceable to The Damned Crack, which is responsible for more bad shooting than half of the bad barrels still existing.
The Damned Crack can come about 2 ways: either from shrinking of the Forearm wood from changes in the Climate, or from somebody removing the Butt without first removing the Fore-end, which simply breaks it out. In either case, The Damned Crack results. It is a crack in the rearward end of the Forestock, right by where the Trigger comes up to engage the Sear. The wood at that spot is very thin and may be cracked-out rather easily. Twisting the BUTT BOLT can open up this crack and likely this is today the most common cause. Back when the rifles ware in Service use, the Crack was less common and generally resulted from the wood drying out, pressing against the metal and eventually opening up. In either case, it DESTROYS your bedding job simply by loosening off the wood-to-metal fit.
You can spot The Damned Crack by removing the Forestock and looking at the rear end of it. You are looking for a thin (generally hairline) VERTICAL crack, right at the back end. THAT is the little monster.
The Damned Crack can be REPAIRED for less than 50 cents, plus an hour or so of your work. Remove the Forestock, wedge the Crack slightly open, degrease with a shot of brake-drum cleaner and allow to stand and dry. When the Crack is clean and dry, you work a bit of 2-part epoxy into the Crack (I use Brownell's Acra-Glas Gel), clamp your repair lightly with a padded C-clamp, allow to stand for the curing period, clean up the repair and reassemble the rifle. MUCH cheaper than a new stock.
About 90 percent of the time, this simple repair will cure vertical stringing in the SMLE.
I have a 1918 Lithgow SMLE here which was shooting a 14-inch group at 100, off the bags. This was an original First World War rifle which had been bashed-about but had never been shot; the BORE was NEW. Repaired the bedding, INCLUDING The Damned Crack, brought the bedding up to Bisley standards (I had the former Captain of the Canadian Team to tell me what to do)..... and the rifle, with the same ammunition, started shooting half-inch groups. I have NO complaints.
It WORKS.