Morning Richard,
I recently purchassed a solar charger and although i have used it these past few weeks it's difficult to say whether it's any good for when the car is left for long periods. I will be going away for a month later in Sept so that will be a true test of it's ability.
Anyway here is a link from amazon to take a look: https://aax-eu.amazon.co.uk/x/c/JIZfbp4BSFq-wEyNOmSbh04AAAGRpx4oXwMAAAH5AQBvbm9fdHhuX2JpZDMgICBvbm9fdHhuX2ltcDEgICCaDDQz/http://www.amazon.co.uk/dp/B0CDGX2MYK/ref=syn_sd_onsite_desktop_0?ie=UTF8&psc=1&pf_rd_p=5d943da7-b45b-4f0a-b413-7648ce2a1b78&pf_rd_r=AF0QMHTEHP2878RZR4XS&pd_rd_wg=M5sv6&pd_rd_w=G7sXO&pd_rd_r=5efe37c6-7667-4712-bb44-80d96ff86ee7&aref=Aq4z8PyH7I