Firstly, you're correct that all is not well. I'm somewhat embarrassed to quote myself, but this
is the sort of distance you should expect before a new chain needs retensioning. That's 3/32" chain, so 1/8" should be significantly better.
Your question is very definitely not a stupid one. It is a simple question, and doubtless the answer will turn out to be simple. Nevertheless it's very far from simple finding what that answer is.
Various random thoughts: -
Chainring bolts - if they're too long, tightening with an allen key on the outside & the special nut tool on the inside might not have clamped the chainring against the spider. If you can't turn the bolts with an allen key & no nut tool without a lot of force, that looks like an unlikely cause. Is the chainring new?
Axle slippage looks unlikely with a drive-side chain tug & your tightening technique. A 1970s Holdsworth is most likely to have Campag dropouts. If they're not chrome plated, slippage is very unlikely, unless the track nuts are bottoming on something in the axle spacers, or maybe something on the inside of the dropouts. That ought to be visible with careful scrutiny. Symptom... have you checked whether the tyre/rim has moved laterally between the chainstays? RHS slippage can slacken the chain by the amount you describe with only around 5 mm of lateral movement at the rim.
Slippage of LHS of axle looks unlikely. On my 1980 touring frame, the tyre would be rubbing against the RH chainstay with the amount of slack you describe if it were a LH axle issue. Symptom to check is the same as for RH axle slippage.
I've had one chain that measured short of 1/2" pitch when laid out on the workbench. That stretched to a stable length during the usual installation & tensioning routine. That also seems a very unlikely cause.
Bearings... Is bottom bracket tightened securely? Tightness of RH crank on the BB spindle looks unlikely if it's a square taper chainset. Is it? Rear hub bearings look unlikely.. Any lateral play at the wheel rim?