I drive an opel/vauxhall insignia. Often when i come to check the oil i find the oil level is above the max rating, I believe the oil level is also somehow raising (possible diesel slipping past the rings?) between oil changes, as I doubt the three garages it has been to have all been as incompetent as to not put the correct amount of oil in. Should i be concerned?
It depends on how much above max. You can read what happens when the oil level is excessive here.
Make sure the oil level is actually increasing. Hot oil expands and will show you a higher reading, which is why usually oil level is checked when the engine is cold.
If it really is increasing, on diesels this is usually caused by fuel getting into the oil, as you suspected. A common culprit is unsuccessful active DPF regeneration - if you fail to complete the cycle the extra fuel injected to increase exhaust temperature will not burn and drain to the sump. Doing this constantly by driving only short distances will accumulate diesel in your oil. A slight increase might be more or less normal, but if it is increasing so much that the level goes above max - you might have more serious issues going on, like a leaking injector. Either way, you probably have diesel getting in your oil somehow and you don't want that.
Diesel is a poor lubricant compared to motor oil and diluting the oil too much might kill your engine due to poor lubrication or even an engine runaway in the long term. Get this fixed as soon as possible.
It might also be coolant, however this is usually obvious and less likely. Coolant in your oil will cause it to develop brownish milky sludge when checked on the dipstick or under the oil cap. In this case you would have even more serious problems, such as a blown head gasket or a cracked/warped head.
An oil analysis will tell you what's in your oil if you're not sure.