Erm yeah they do i think, its not very high just 13KG but i cant afford a really expensive one that goes RF longer, The only difference that i can see seems to be
well this is what it says about plus
* Rearward facing for newborn babies (birth to 13kg), then forward facing for children from 9kg to 18kg
* Click & Safe audible harness system is designed to aid you when securing your child in the seat
* 5 point safety harness
* Side Impact Protection with deep, softly padded side wings
* Unique pitch control system reduces forward movement in the event of an accident
* Performance chest pads
* 5 recline positions the seat adjusts without disturbing your child (when forward facing)
* Newborn insert provides extra comfort for the younger child
* Extended recline position and newborn insert create a more natural sleeping position (when rearward facing)
* Spring loaded belt locking mechanism allows easy installation
* Slim base for a better fit in cars with sculptured seats
* 2 seat belt routing options (when forward facing)
* Harness retainers make it easy to get your child in and out of the seat
* Quick-remove cover without interfering with the cover
* Softly padded and washable cover
* 3 point belt installation (rearward and forward facing)
* Spare cover available separately
* Weight: 7.5kgs
* Dimensions: 66cm x 45cm x 57cm (HxLxD)
and this about Si one,
* 5 point safety harness with one pull adjustment
* Side Impact Protection
* Performance Chest Pads
* Rearward facing for Newborns (birth to 13kg)
* Forward facing for Children (9kg to 18 kg)
* Pitch Control reduces forward movement in even of an accident
* 5 Reclining Positions (forward facing only)
* Extended Recline Position and insert (rearward facing only)
* Spring loaded belt locking mechanism for easy installation
* Alternative belt routing for fitting in wider range of cars
* Newborn insert cushion provides extra comfort
* Conforms to R44/04
But the Plus is about £40 more...