My Insurance Company says that the replacement cost of my house is about three times the market value. They say I must insure it for 80% of what they say is the replacement cost or they will not completely pay for partial damage.

How can I prove to them that their replacement cost is not correct?

Get quotes from other companies for a start.

Check estate agents for similar sized/location properties.

There is always an excess that insurance companies don't pay. For example in my case it is the first 100 GBP in any one claim.

Something sounds fishy here. The market value of a home includes a number of factors; the land, location, and building costs. Replacement value only includes building costs and the replacement of furnishings.

I would say its just about impossible for replacement costs to exceed market value let alone be 3x it.

Yes, go to a couple of local builders and ask them what it would cost to build a home comparable to yours (sq footage, # of rooms, # of floors). Then do a home inventory of all your furniture, appliances, clothing, etc. Include estimated costs to replace those items. (You should have such an inventory for insurance purposes anyway).

Then take that to your insurance company. While you are at it get quotes from other insurers as well.