1) Is something like a Nano Ledger or a Trezor the right method?
2) What app should be used?
Is there another more definitive, secure method of accumulating BTC for long term holding?
Number one is the right method for safe bitcoin. I think you can buy one of the hardware wallets recommended to store your bitcoin. You don't have to always check or connect the hardware to the other devices if you don't want to sell at the current price. If you want to store for the long term, you don't have to worry about the price.
Number two will not be necessary if you already have a hardware wallet, but you can use apps to store a small amount of bitcoin and use it for another purpose besides storing the bitcoin. Trading will be a good method to accumulating more bitcoin, but you should have the skills to make a profit.
I want to regularly add more to the wallet so an easy transfer method.
I currently use a desktop PC with an external HD and internal SSDs.
Should I buy a laptop on it's own, and download nothing but electrum on it to store the BTC?
Would using a VPN be sensible when accessing the internet from said device?
A hardware wallet is something like Nano or Trezor?
Thanks again - very new to the wallet side of things, so far just left the small amount of BTC on the exchange, which I know is stupid, but I only have nothing relatively.
You don't have to buy a laptop if you already have a desktop PC because you already have a hardware wallets such as Nano or Trezor to your PC. Using VPN will not be a problem, as long as you know what website you visit, so there is no need to worry.
You can use a small amount of bitcoin to trade on the exchange so that you can make a profit from trading. But you should have skills to trade because, without skills, you will not be able to make a profit.