The best time to visit is undoubtedly during the summer months as the weather is more settled and temperatures are warmer. The hottest temperatures may occur in July and August, but this is also when the schools are on holiday and attractions can be very busy.
May, June and September are worth considering as the crowds will be smaller and you still have a chance of enjoying some good weather. Maximum temperatures in the middle of the summer average 20°C, although this can vary widely.
