Manc Man - don't bother getting more lenses yet. You will only confuse yourself even more, the lens coverage you have is ideal for getting to grips with any kind of photography that doesn't involve shooting at long range (motorsport or wildlife in general) and it will produce really good pictures if you keep it around the middle of the aperture range (f5.6 - f11 or f16)
Landscapes are more about the scene AND THE LIGHT. The most important aspecrt of landscape photography is being there, and that means PATIENCE, not kit. If you look at the greats like Joe Cornish, Colin Prior (especially Colin!) Charlie Waite and all the rest - they use very few lenses, but their feet, a lot. Most of them use 2 lenses for 90% of their work, semi wide angles like a 35mm equivalent on 35mm. They do use filters though, to manage the light (see, that word keeps cropping up - LIGHT, it is all about light, not lenses).
A flash is not really going to be that important for landscape/seascapes either - in some instances you can lift elements with it, but not the whole scene. The whole scene relies on natural light - that means being there at the right time, or recognising what you can do at the time you are there. A few filters and learning how to use them will be far more important than a flash or new lenses - a polarising filter would be more use to you (get a reasonably good one, not an el cheapo, a cheap one will introduce colour cast.) Expect to pay the same for a polarising filter as a flash gun.
A tripod - again, get a good one and buy used, you will get 10x the tripod for the same money....stand by to hear from all those photographers with a spare tripod to offload.

A tripod can't go wrong, it is only 3 legs and a screw fitting with a clamp head on top. Don't go for a pistol grip thing - just a 3-way head. Ball head if it is a good one. A spirit level built in is useful, or take a small one with you (horizontal horizons!)
You have the basis in your camera and lens, you need inspiration and understanding more than you need more kit.....spend £50 on books and soak up all the technique help you can. The best bit of advice I was ever given as a starter was:
It is better to come back with ONE good or outstanding picture than a whole bagful of film full of rubbish. That applies to any genre of photography, not just landscapes
I need another hot whisky!