You don’t want ground rent which will exceed 0.1% of property value at any point now ….as rightly or wrongly, that’s the condition for many mortgage lenders and it’s not just about getting a mortgage now, but people getting a mortgage in future.
Yes, you might be able to get a deed of variation and they might consider it, especially if they’ve had other buyers drop out for same reason. However, this will cost the seller, so they may resist and hope instead to get a cash buyer who might not be so bothered about it. It depends if it’s the kind of property an investors who do buy to let might be interested in.
An alternative is to find out the cost of extending the lease via the statutory route, which will automatically make ground rent zero. You can then reduce your current offer by that value plus a bit for inconvenience. Currently you need to have been there 2 years to carry out the lease extension, but you could then use the money to extend the lease and deal with the problem, using that money. Of course you’d have to ensure it wasn’t spent on something else, and it would involve some hassle. If it’s a long lease remaining, you might feel you could wait until further leasehold law changes come in…prob won’t be very soon.
To be honest, probably best to walk away.