I agree with Jose
Really there is almost no difference between 1 inch and 25mm. Not many people that could tell the difference by just looking. I think the shop you are dealing with probably has 1 inch stock availabe and not 25mm. They are trying to sell you what they have, not what you need. I am almost sure the 1 inch stock would work fine, but if it was my boat I would use 25 mm stock. Also the price of the stock is just a small percentage of the cost of the total job. If you tell them you want to use 25 mm stock they should be able to get it for you. (We are talking about a piece of steel here, this is not something had to find.) If they won't make the effect to find the right steel I would look for a new shop.Good Luck.--Dan