I'm considering the above package which includes Tannoy Mercury v1 speakers for £550. Would it be worth upgrading the speakers or are the better deals / makes to consider?
An excellent combo but it deserves better speakers.
Although I've not heard the V1's I owned their predecessors the F1 Customs, and excellent budget speakers that they are the Marantz will drive far, far better. A good starting point to listen to would be Dali Lektor 2's. The Monitor Audio range are also a well recommended pairing, although I've not heard them personally.
Another recommendation for the Dalis here. Excellent speakers and the next level up from there is the KEF Q300/Monitor Audio RX1. You should also try, as a competitor to the Dalis, the MA BX2.
If you want to keep the budget tight the Wharfedale Diamond 10.1 work well with the Marantz kit.