I'd like to say I got a stonking deal until I realised a trade in error and cancelled, but in the spirit of sharing, some tips.
1. EPP Site if your employer supports is cheaper
2. Check your email for the pre order code (case, screen protector)
3. Trade in shows a £420 default but goes upto £690
4. 5% off if using the Samsung shop app
5. Claim the free watch 6 40mm, sell, use or gift
Cashback sites are hit and miss, but worth trying Topcashback or Quidco.
