Upvote:2
The cheapest available ticket that permits overnight break of journey is an Off-Peak Return (not an Off-Peak Day Return), costing £58.10. However, when you break journey overnight you must recommence onward travel before midday the next day, so that wouldn't work in your case.
I'm not sure why you'd think that two separate tickets is dearer - two separate tickets cost £21.60 + £26.80 (Off-Peak + Anytime), which actually works out at less than an Off-Peak Single for your journey (£51.50). The only cheaper tickets are Advance, which tie you to a specific service.