When I got the print back it was so dark as to be unsellable. Rather disappointed I contacted the printers and they said I should have used an ND filter.

An ND (neutral density) filter would have made your image even darker, so it's hard to see how that would've solved your problem. Perhaps they meant a graduated ND filter, which could reduce the brightness of the sky and allow a longer exposure for the rest of the shot.

Perhaps I am naive but I thought that a printer should be able to create a work equal to that of the jpeg.

If you sent them the JPEG image, that's exactly what they did. The issue is that the same image appears differently when rendered on paper than it does on the screen.

To be more succinct I just want to know how to get my jpegs to look the same in print as they are on screen.

That's actually a really big topic, and something that people spend a lot of time and money on. There are calibration tools that you can buy to adjust your screen to a particular standard. To do it right, you'll also need a color profile for the printer used by the service provider.

There's no substitute for a properly calibrated display, but if you're not ready for all that then you can get pretty far with basic trial and error. 4x6 prints are usually quite inexpensive (20¢ or less), so they're a good way to experiment. You can try saving the image with a variety of changes and printing the entire batch, keeping track of which image has which changes. It helps to add a little identifying text to each image so you can tell them apart. Also, talk to your provider and ask them for tips to improve the way your images look on their equipment.