If problem persists after a battery change, the starter motor may be getting a little tired ...whilst it may not yet be worn out, the contact faces on the coil / and brushes may be dirty making the starter take additional current and resulting in its slow action of turning the engine over and dropping the voltage to the spark plugs.
When you look at the starter motor it will have a cylinder on the top, this is called the coil.
This normally has a live feed to it from the battery.
When you turn the key to start the key, this energises the coil, which is magnetic so pushes the coil over which makes electrical contact and allows the battery to feed the starter motor & turn the engine over until you release the key.
The starter motor is normally a ' lucas ' item which is quite commonly found in the UK.
http://www.ebay.co.uk/itm/S190-Rover-Mor...=item337f995dc9
