That seems very cheap at 8.5-9k. I had a quote of £13 for less than 4kw and only 5kw battery. Who are you using?
And I would expect @DaveNN to have paid VAT on his combo where as who ever quoted you should be VAT free. Yet again goes to show that where the government funding goes...To top it off most are just interested in flogging you into a g98 installation as to milk it even more (faster whilst the grant exists)
Trade Sparky for my Sofar Battery Inverter
HDM for my batteries
Solar Energy Stores for panels etc and Solar inverter.