Not sure what the first one is (Rimmers online epc doesn't recognise the number) but I would think it is a different gasket (unless the LBJ is a typo and yuo meant LKJ).
LKJ100110L, LKJ100110SLP and LKJ100110 are all the correct gasket for the K series plastic manifold.
The L suffix usually denotes an aftermarket part - usually a lot cheaper (around £3-£4), but probably not as good quality.
The SLP suffix stands for 'Service Line Part' which is like a second string cheaper option genuine MGR part from XPart (about £7).
The part number with no suffix is a genuine MG Rover/XPart OEM part, is more expensive (about £18) but should be guaranteed in quality and fit. The original green gasket as used in the factory has been unavailable for a while and has recently been replaced by a new improved black gasket made of viton (under the same part number), and that is probably the best one to go for.
I have had the same kind of repeat failure problem as you on my ZR, and had been using the SLP version (as that was the best available from XPart at the time), but have just yesterday fitted one of the new viton ones, so we shall see if it lasts any better. It is slightly thicker and stiffer than the SLP one which is perhaps a good sign. (interesting to see on the label it is made in India)
The viton one looks the part, but is a bit of an unknown quantity at this stage. Of the other gaskets, the Service Line one will be better than any of the cheap aftermarket ones, or there is one available from Payen which seems to be highly regarded (Federal-Mogul/Payen supplied the original gaskets to MG Rover).