Bad Grid voltage

Spain, 230VAC supposedly. I have a grid tied system with a 48v lead acid battery bank. (24 x 680 ah 2 volt Hoppecke batts)
One 2kW array on a single axis tracker and a second 5kW array fixed. The arrays connect to my Victron Quattro 8kW inverter via Victron mppt charge controllers.
My grid connection is inadequate. High summer the voltage can fall as low as 180v and during the week in the winter it can be as high as 250v. Sending back my surplus in the summer will push the low voltage to semi acceptable levels but in the winter i have to turn off export or i hit 260v in an instant. Today, in August, I simply have my grid breaker open.
Any ideas for something i can do here on the property as getting my electricity provider to rectify their issues is simply not going to happen.

If the electricity provider is not willing to rectify the issue at their end, then there is really not much you can do to fix this.
How far is your inverter from the electricity grid connection point and what size cable is used? If the supply cable is very long and a small gauge then it will compound the problem.

What is the solar feed-in-tariff (FiT) in Spain?

Thanks Svarky!
Good call on the supply cable observation! When the grid feed was run in to the house by the previous owner, house had been “Off Grid” for ten years prior to this, the contractor made a complete mess of the install. The first cable he installed, trenched and back filled into my concrete 200m driveway had to be dug back up and replaced because it “didn’t work” for reasons I am unaware of. Replacement cable does “work” but could easily be undersized as you suspect. However, supply in general to the village is poor with frequent black outs. I’m no electrician but if part of my problem was a high resistance connection to my property wouldn’t that be proved wrong by the fact that I see high voltages, 250v, during the winter when there is low demand in the village?
Feed in Tariff is 0.05 cents per kWh which hardly makes it worth bothering as far as I’m concerned. (I pay 0.28 cents per kWh that is take from them)

We run into the same problem here in PH except we don’t get the high voltages, only low voltages during hot season then back to normal when it’s cooler. The grid tie inverters shut down and the off grid inverters won’t switch back to utility. I had to readjust the settings on my inverters for them to work. Here the problem is inadequate transformers. They have connected too many drops to too few transformers and the transformers are also too weak. We are buying new transformers for our larger building’s connections and having them installed by the electric company, they won’t provide them. Here it’s also a problem nationwide, so it could be the same in your case as well. the dog