A very interesting report from the Polling Observatory which looks at recent trends in voting intentions: How to read the polling tea leaves
In particular there is a helpful discussion of 'house effects', i.e., the tendency of a polling company to report high or low figures for a particular party. However,if a pollster tends to show one of the parties doing better than the polling industry on average, it does not automatically mean their estimate for the other main party will be lower than the average.
Prompting for the Brexit Party and controlling for past vote appear currently to have significant impacts on poll numbers. In the former case, pollsters that prompt for the Brexit Party in their surveys tend, unsurprisingly, to report higher numbers for the party.
The use of past vote (i.e. how people voted in 2017) to weight samples to make them representative is a longstanding practice in the polling industry. However, this can introduce error through people misreporting their past vote, leading supporters of a party to be overrepresented in the poll.
They wisely conclude, ' There can be no way of knowing which pollster is right before election day, but it is worth urging some caution in how these sorts of numbers are interpreted by those in politics, media and the wider public.'