I have no experience getting lapped so i can't help you there

But, i can say, if i'm coming up on lapped traffic, i just prefer them to make their intentions obvious and early enough so i can react and pass cleanly. Usually if you're being lapped, you are typically slower than the guy coming up behind you right?
I respect the lapped cars though, but if they are way off pace, it does get difficult if i'm in a heated battle.
There are courteous ways to allow people to pass you (for example, coming out of a corner, get off line and get on the throttle a little later than normal and the following car should go right past without it causing you to lose much time; or vice versa, get off line and brake a little earlier than normal going INTO a corner). As David said though, you don't necessily have to get off line as sometimes the overtaking car is planning to go to that space as well! You just have to understand where you are on track and what line people are taking. If speeds are similar, you don't have to let them by easily, just run your race. It's more interesting that way. Just don't hold people up for too long if it's obvious they are much faster than you.
Spa is a very wide track though, so it shouldn't be a major issue like it is at other tracks. Just don't get all nervous and end up crashing by trying to go too fast for what you and your car can handle just because you see someone coming up behind you. Plan ahead, that's part of racing.
The 7 day weather forecast summary for Spa Francorchamps Circuit:
The forecast for Spa Francorchamps Circuit in the coming week is predicting the average daytime maximum temperature will be around 10°C, with a high for the week of 18°C expected on Tuesday afternoon. The mean minimum temperature will be 4°C, dipping to its lowest on Friday morning at -1°C. Expect the next week to have some days seeing a little precipitation and some days with rain, sleet or snow.
The indicators are that Saturday will have the most precipitation with an accumulation of around 11.0mm. On the whole winds are likely to be moderate.