On the advance fare front, i can find a 0705 departure from Sheffield direct to Reading at £19, but the return is more problematic. There are some £11.50 fares Reading-Birmingham New Street but they just miss the last departure to Sheffield at 2103. There are also some advance fares for Birmingham-Derby later on for £5 but they also miss the last departure Derby-Sheffield. Given it's a Saturday the services finish surprisingly early on the MML.
...And as it's a Saturday, there's going to be very little chance of getting MDS and MES type fares I'd imagine! That MCS for £19.00 is a good deal, but it'd be trying to get any relatively discounted seat on the way back that'd be near impossible...As ye rightly suggest!

hock:
I presume ye've already looked at Advance tickets on the direct AXC trains between Reading and Sheffield, havn't ye?
I would say though the LM route via London is unlikely to be practical given the return leg Derby-Sheffield would be tricky. The XC route would probably be better, although more expensive.
One thing that's just hit me is that the £45.80 route that I came up with - Though a fiver over the cheapest - Would allow Sheffield Vic to complete his entire journey on a single direct train with no complicated routings and changes required!
Unless things have changed a bit since I was last on the Oxford line, the XC services between Newcastle and Reading call at Sheffield, and all of the splitting points I've highlighted for the route...Meaning that all three splits could be used on the one working (In full compliance with the NRCoC) in both directions! 8)
Anyhow...I've been up so long, my brain's now splitting at Dalwhinnie, Canolig Caerdydd, and Ashford Intl
all at once, so I'm off to bed. Hope this has helped ye a bit, Sheff!
>> Death <<