Hi Fringe, I have had the same problem. I have had a 3m dish for 5 years now, three of them!. The first one, a metal one, lost the signal totally for 20 mins. each day at approx. 21:30 in late October and also in March. Now I have a fibreglass dish and a Grundig quad LNB and the reception is better but I think it is down to dish alignment. I am now viewing via a Dreambox which gives individual signal strengths on each channel. During the morning signal strength on BBC1 is currently upto 52% but by 21:30 this has dropped to 38% and the picture starts pixelating however within 15 minutes the signal strength is back upto 52%. The main issue, apart from being so far South, is the suns position washing out the already weak signal. Also with the very quick recovery in signal strength maybe Astra are moving Astra 2d or changing something?
As far as your dish is concerned I would recommend attaching 2 wires, one each side of the dish, with an adjuster to tighten/loosen the tension, readily available at your local ferrateria. Just half a turn on the adjuster, the equivalent of pressing gently on one side of the dish, can vary the signal strength by 10%. Then have a play with varying the tension i.e pulling the dish one side or another and optimise the dish. Repeat this exercise with the LNB left/right & in/out to maximise the signal strength hopefully to such a level that when the sun weakens the signal you still have enough strength to prevent pixelation. I hope this helps but if you need any more advice I would be pleased to try and help.