I'll have had my van from new for 6 years on 1st March. At the mo' it is on 33,000 miles in 70 months. So that's 471 miles per calendar month. I know that 1000 miles of that was going canoeing in Scotland last year and another 2000 would be for visits to the Lakes and North Wales over the years.
So actual work use is about 30,000 divided by 70 months which equals 430 per month so about 100 miles per week.