Hi
Im blocking up a garage operning and would like to know if there is any building regulations as to whether I can use a conncrete lintel at the bottom toothed into the piers at each end to then build my wall off the operning is 2100mm wide and I'm looking to install a 100x75 at 2400mm long lintel, any addvice will be most welcomed

Building codes are always local. Since you are talking metric, I assume you aren't in the US. Ask your building inspector. Your lintel sounds OK as I read it, but it also depends on the load above, how many stories the house is (assuming the garage is under the house) and how the load is distributed down.

Thanks
I am from the uk
There is not much weight bearing on the lintel as there is a lintel already at the top of the operning taking the main weight so it will only be the weight of the blocks I use that will be bearing on the lintel

Oh right, at the bottom.. so it depends on how the foundation was made. Just piers, no frost wall? Then the whole wall might heave when the ground freezes.