toddler seat for pram?

[fernie3]

im a few months I will have a newborn and a 16/17 month old (17 months on due date but my this is my fourth and my last two have been a month early).I HAVE a double but I have also seen toddler seats to go on top of a pram like this :

www.amazon.co.uk/BabyDan-Pram-Seat/dp/B000N5NC1Q?tag=mumsnet&ascsubtag=mnforum-21

I have a loola buggy but Im not sure if the carrycot would be suitable also im not sure how safe they are?! I had one like this when I was a toddler but it was on a GIANT pram lol.

has anyone used these? It wouldnt be all the time but my current double (out and about nipper) doesnt fit in alot of the shops here (small town with old sized doors) and doesnt fit through my front door so for just nipping out or to playgroups i was hoping for this sort of thing. I had a buggy board last time but i hated it as I foudn I wa shaving to lean over it all the time and I was getting a really bad back! Plus my daughter would be a little small at 16/17 months to use this - she only just started standing and has yet to put one foot in front of the other lol.