Thanks "collectors" , that is what i thought ... i will slide out and check how much of the "space" i have to play with when i consider the new purchase.
A little puzzled by your comment "plumbbob" , i am only a naive non-teckie but always assumed a double oven was one above the other fitted as one unit up to approx shoulder and chest height and that "built under" meant only up to the level of the worktop surface ?
The present unit is electric as will be the new purchase.... present configuration is classed as a large oven with small over and grill above.
Re the screws there are actually 8 very small screw positions but only six were used.
Thanks you both for your help
Built in ovens are designed to fit in a tower unit where there is no height restriction so the main oven will be larger than the top oven. A built under cannot be greater than 720mm in height overall so both top and main ovens are normally pretty much the same size.