There are two separate issues here:
• "they seem to have a lot of flare coming out of the planet like an ‘X’ where the planet is at the centre."
This is normal for all Newtonian reflectors, so much so that most of us don't even notice them. They are diffraction spikes caused by the spider which supports your diagonal mirror. They are inherent in the telescope's design, and the only way to eliminate them is by going to a different design: a refractor or a Schmidt or Maksutov design. But, don't worry about them, as they actually have absolutely no effect on the main image, except for a slight loss of contrast.
• "they seem slightly blurred (almost impossible to get a sharp focus)"
This is a different issue, and springs from two different causes. First, the telescope may not be collimated properly. Collimation is the process of lining up the various optical elements in a telescope. Collimation is a normal part of the maintenance of all telescopes, and is not difficult if approached systematically. The process is described well here:
http://www.backyardastronomy.com/Backyard_Astronomy/Chapter_15__Polar_Alignment,_Collimation,_Cleaning_and_Testing_of_Telescopes_files/Appendix%20B-Collimation.pdf
The second factor is in the images themselves. At present, both Venus and Mars are far away and, as a result, show very small disks, 22 and 13 arc seconds respectively, as compared to Jupiter, 34 arc seconds. This has two effects. First, any detail on these planets is vey much smaller in size than the detail on Jupiter. In fact, no detail is ever visible on Venus except for its phase (slightly more than half). On Mars you may see a tiny polar cap and a faint smudge or two on the rest of the disk. Secondly, their small size makes them more subject to the degradation of "seeing," turbulence in the Earth's atmosphere. As a result of these two factors, seeing detail on Mars is a challenge even in much larger telescopes than yours!
Finally, there is the question of your eyepieces. Planetary observing is probably the most challenging aspect of visual astronomy, because the planets are so small. The planets require much more magnification than any other object you're likely to look at, except for very close double stars. Your eyepieces give you 26x and 65x, whereas serious planetary observing begins at around 150x, and is mostly carried out at 200x to 300x. The short focal length of your telescope, while providing fine wide-field views of deep sky objects, is not well suited for high magnifications. The shortest focal length eyepiece commonly used, 4mm, will only get you 162x, which is only barely adequate for planetary observing. Even then, the small aperture of your telescope may preclude using this high a magnification.
Don't waste time or money on filters. They serve no useful purpose on a telescope as small as yours. You would be better off spending the money on better quality eyepieces than those which came with your telescope.
