I haven't been uploading as prolifically as to begin with and this is due to a number of reasons including having to deal with a burst pipe at home. However, today's update contains a number of improvements which should improve the overall look and feel of the game.

At this point I'm torn between making an endless runner type game where it just gets tougher and tougher, or having proper levels and missions. 

Changes

Enemies turn gracefully to follow you, rather than snapping around - this really improves the look of the game and will be a variable as the difficulty increases. This code will also be used for my forthcoming homing missiles.

Ditched the cakes, now you rescue soldiers - this makes the game feel less childish

Added a prototype second level with roads (but no vehicles as yet...)

Added road and building tiles - also created a tank sprite, but still in the midst of coding its behaviour

Palm trees are now tiles and not sprites - efficiency improvement

Player can't respawn inside another object

Added sound when baddies fire

Rewrote a significant amount of the code to make it easier to extend