What was your opinion of the Yamaha and did you use or enjoy any of that receiver's propriatary DSP Programs?
If you've the budget to facilitate an upgrade as opposed to just an update then also consider the new RXA1070. This model will be the first Yamaha model below Yamaha's flagship models to be able to apply DSP Programs to both Atmos or DTS:X audio. THe only real downside to it is its price and the fact it is only a 7 channel AV receiver.
Given your proposed budge then I don't think you can go wrong with the Denon AVRX4300. THis is a 9 channel receiver with 11 channel processing. THis gives it the potential to be at the centre of a 7.1.4 Atmos setup if you add 2 additional channels of external amplification. It also includes most if not all of the features you'd get on DEnon's higher tier models including dual main room mirrored HDMI outputs plus a third dedicated second HDMI zone output. THeAVRX4300 can be had for £800 Denon AVR-X4300H AV Receiver
You may even be able to trade in your RXA810 against the purchase?