J&K wrote:
I am thinking of putting a new patio in my back garden, over the
existing slabs. Whilst this is okay from a damp course perspective the
existing drains and grids will obviously be 1 1/2 inches below the
level of the new patio once I've finished.

Is there an easy way to raise the level of them?

Thanks for your help.

Firstly, I seriously doubt that your new flags will only be 1.5 inches
higher than the existing ones, you do realise that they need to be bedded
onto sand/cement for levelling purposes? - if they are just placed on top of
existing flags they will simply slide about, rock and generally be a pain in
the arse until you eventually thrown them into a skip....but that is up to
you and you didn't ask for advice about that - your grids are currently
level with the surface of the flags? - this is never a good idea and I
always put drains lower than the finished surface, or if Im laying flags /
concrete / block paving, I aim to finish at least 2 - 3 inches above the
grids and then bench strong sand/cement around them and make a gulley into
which surface water will run...so to cut a long story short, your grids are
already bordering on being too high, don't make them worse by raising them
up, it doesn't matter if they end up six inches below the surface of the
drive / patio etc, water always finds the lowest point.