The invoice or ex-showroom price is what the car cost new. The IDV is what the insurer will pay if it is stolen or written off: today's list price minus a fixed depreciation, 5% in the first six months and 15% by the end of the first year, rising to 50% by five years. IDV also leaves out road tax, registration and insurance, which is the gap return-to-invoice cover fills.
See on-road prices, with registration, road tax and insurance included