The OG is unlikely to have been 1035 with the 1.8kg kit and the fermentables you've added. More likely to be 1045 if you filled it to 21 litres, but it can read lower if it has not been mixed evenly (not always easy to do with DME, but should ferment out fine). So your alcohol/attenuation is higher than you think, but the FG is obviously still too high.
The yeast you get with most the 1.8kg can Muntons kits ('Muntons Active') is really designed for use with sugar as 100% of your additional fermentable. This is why they recommend you buy 'Muntons Gold' yeast if you are using DME or LME (although they have stopped selling Muntons Gold yeast!).
I've used the 'Active' yeast you get with the kit successfully with brews made with 1kg of DME, but always added an additional packet of 'Muntons Active' yeast, so pitched double the amount (but they have also stopped selling this too!).
So, it sounds like the the yeast is not really up to the task at hand, as others have said. This may particularly be the case if the beer kit in question has been sitting around at room temperature for a long time, as the viability of yeast decreases over time and the amount provided in the first place is not great.
