Pibber, Your bike, if original, has a 6S freewheel hub, not one that uses a cassette on a freehub. To use the 105 integrated shifters you'll need a new rear wheel, or a wheelset to match wheels. This is a common modification as the 130 mm spaced hub fits well into your 126 mm spaced rear dropouts without any modifications providing there is no pre existing defect in the rear triangle. Test your existing derailleurs for compatibility with the new shifters.
Brad