Wickes might keep bags of tarmacadam, if it is tarmac. Clean out hole of all particles, and pack down as tightly as possible and level off.
If concrete, same as above, mix a strong ratio and trowel firmly up. Trowel over tightly when water disappeared from surface.
Also, before putting the 'Instarmac' or whatever in, try and make the sides of the hole as vertical as possible, so it gives a good edge. If you leave the sides 'sloping', the tarmac will start picking out again. This may take a bit of time with a hammer and bolster, but as with most jobs, the preparation is the hard bit!
In that case, just square it off as best you can. A square/rectangular patch will look a lot better than an irregular shape which will look like you have just blobbed a lump of tarmac onto the drive.