Know Your Daleks No. 248

The Recon Dalek.
Daleks lack subtlety. If they want to take over your planet, they simply turn up in a fleet of spaceships and invade. However, this was not always the case. In the past, a special Reconnaissance Dalek was sent out in advance of the invasion force to check out the planet's defences and suitability for exploitation, and to report back before the fleet made its move. These Daleks went so deeply undercover that no-one had ever heard of them for the last 55 years.
They had a number of special abilities which made them a formidable foe, but they tended to be prone to defeat at the hands of primitive societies armed only with swords, spears and axes. The Recon Daleks had been trained to deal with laser weapons etc, so when one of them encountered an army of Anglo-Saxon warriors in Dark Ages Britain, it got its backside whipped. These warriors decided to cut the Dalek mutant into three parts and bury them in parts of the world that hadn't actually been discovered yet. This amazing foresight extended to them realising that the three bits of dead Dalek might just conceivably come back to life again and magically transport themselves together. Their amazing foreknowledge did not stretch as far as planning any contingency should one of the guardians of the three bits get killed before they could reach their destination. Nor did they bother checking in with this part of the operation to make sure that the bit was actually being guarded. No-one questioned the lack of a Christmas Card from Yorkshire in 1500 years.
Back on Skaro, meanwhile, the Emperor Dalek decided that the Recon Daleks were rubbish and a waste of resources, and so discontinued the programme.
The descendants of the original guardians were mightily peeved when they realised that they really should have checked up on the Yorkshire guardians sooner, as some archaeologists dug up the British bit in January 2019 - the only archaeologists in recorded history not to be in the pub on a Bank Holiday. Recon Daleks merely needed ultra-violet light to spring back to life and teleport their assorted bits back to base to become reanimated.
Unlike other Dalek mutants, they could take over the mind of a person whom they physically latched on to - which was handy for getting around.
The person would then be compelled to kill people and to help build a new shell for it.
Recon Daleks were programmed to seek out the most useless materials to build their new shells, in the event that they were ever hacked to pieces by primitive warriors, armed only with swords, spears and axes. They were genetically programmed not to seek out the nearest high-tech manufactories of special metals, or any other industries which could supply half-decent shells.
In order to fulfill its mission, knowing that its fleet would have been hanging around in space for 1500 years waiting for its report, only rusty old bits of iron would do for making a new casing. Corroded metals clearly provide the maximum defensive capabilities. If Dalekenium shells could fall prey to swords, spears and axes, then some rusty farm machinery must surely be of superior defence against tanks, machine guns and bazookas. If the farm you raid has some ballistic missiles hanging around, so much the better. These can be housed in the skirt bumps, of which there are only three to each skirt panel, rather than the usual four.
The new shells can be built remarkably quickly - in just the time it takes, say, for someone to hold a conversation with their estranged father in a cafe.
As far as the Doctor is concerned, it really helps if her companions' estranged fathers are carrying around combination microwave ovens on a Bank Holiday - as you do. That's because such ovens contain components which are even more effective than swords, spears and axes at destroying Recon Daleks, and work even better than tanks, machine guns and bazookas.
A word of caution. In the event that the Recon Dalek loses yet another shell, it can still run around and attack people - estranged fathers are particularly at risk. If this happens, simply park your TARDIS alongside a convenient Black Hole or super nova and wait for the Recon Dalek to be sucked into it. Works every time.