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Adrian, an eyepiece in the range 5-7mm will be optimal. A single short eyepiece with good eye relief is generally better than a longer one with a Barlow.
For high power views of small targets (planets) contrast, sharpness, perfect colour correction and lack of ghost images are high priorities. Minimising the number of optical surfaces in the light path is a very significant way to achieve this.
For this reason modern variants of simple eyepieces such as plossl, orthoscopic, RKE or monocentric remain popular. The only real advantages of more complex eyepieces (such as LVW, Pentax, Nikon, Radian, Ethos, Nagler, the ES range...) are more eye relief and wider field of view, at the expense of more glass in the path.
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