I haven't done the sums on this one, for the available light, but I strongly suspect a 50W panel is over kill, and will cost considerably more than the optimum solution.
He doesn't need a sledge hammer to crack this nut.
With respect Prof John, neither you nor I have any chance of doing any meaningful calcs.
We critically don't know about the grime, shadows, bird dumps, snow or dead leaves , some of which will come into play during the winter. Plus we don't know how the weather will be set, but can assume it will not be wall to wall sunshine month on month, more likely overcast for appreciable periods. Clearly we know the sun angle and predicted lighting, but don't know how much of that will actually get through to the semiconductors.
All the above can have massive degrading effects on available yield, a 50 Watt figure in direct standard reference sunlight, bears no relationship to the probable yield in this case.
Achieving an adequate yield becomes very important, because if adding this system, if it fails to yield, it will do more damage than if it was not fitted.
If a battery is just dealing with a healthy level of self drain, it becomes counter productive to put in even a minimised system. If healthy and without parasitic losses, it needs nothing for two to three months
If we are going to do anything, then it could well turn out we need a sledge hammer to crack a nut. Knowing its performance could be very low, over designing is prudent.
Few purchasers are going to be enamoured after investing, to write off the battery because a wet leaf got stuck on it or there was a fortnight of dull weather. IMO, do it well to mitigate losing the battery or not bother.
Apart from paying a few tens of pounds to buy a larger panel there is no real downside to going over the top, after all that overspend is a fraction of the replacement battery cost it could save.
The technical side is well capable of dealing with excesses, a decent controller simply shuts down the solar panel's current output, as and when required.