Since 1997 when the constituency was formed in its current shape Colchester has been an island of yellow in a sea of blue across much of north Essex.

Sir Bob Russell has held the seat since that election 18 years ago and gradually extended his majority on each polling day to its current level, writes Will Lodge.

Most polling organisations have rated the seat as one of the safest in the country for the Liberal Democrats.

Although the national picture for the party is much less clear, with a lot of uncertainty around how the electorate and party faithful will react to five years of coalition government, in the past Colchester has not seen the big swings in national opinion reflected in its own results.

This may well be down to Sir Bob’s own local links to the town as former borough councillor and mayor as well as his position on the backbenches rather than doing potentially more controversial ministerial work.

His nearest rival is likely to be the Conservative candidate who was closest to him in the polls last time around.

Will Quince again reflects the local rather than national picture, leading his party’s group on Colchester Borough Council rather than being parachuted in.

The Conservatives seem to be mounting a more significant campaign in Colchester this time round, with a whole host of big name ministerial visits – including the prime minister twice in the past year – but it will take something special to overhaul the existing majority.

Labour also have a locally-based candidate in Jordan Newell, though without a background on the council he may be more of an unknown to the electorate than his rivals.

They did well in the last council elections, being the only party to make a gain – albeit of one seat – but only led the race for third in the closest ward in the town centre, narrowly coming top in a three-way battle between themselves, the Greens and UKIP behind the Conservative and Liberal Democrat candidates.

So what big issues in Colchester will candidates have to get a grip on if they are to secure victory in May?

NHS policy may well be key, with candidates setting out how their party will ensure reform to meet increasing demand across the board, following a series of headlines around Colchester General Hospital over the past 18 months.

Housing and planning could also be crucial. Although much of the borough’s planned growth actually lies just outside of the constituency, Colchester as a whole is one of the fastest-growing towns in the country meaning how any future government will ensure infrastructure matches new housing is a hot topic.

Defence policy may also be a key factor for the small but significant electorate linked to the garrison, and while the campus itself is not in the constituency many students at the University of Essex do live in the polling area and could influence the vote if higher education issues are not addressed adequately.

For more on the election battle in Colchester, see here