The new series of Derek opens with brilliant comic scenes about spiders, electrocution and sex between clowns and chimps but, by the end of the half hour, viewers just might be quietly weeping.

In the staff toilets everyone crowds in to gape at a giant spider on the tiles. Dougie (Karl Pilkington) is summoned to dispose of it but, on assessing its decidedly exotic state, says 'what do you want me to do? Deport it?'

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More jokes swiftly follow. Derek's father reminisces about travel, telling his son that 'nookie' is the only reason to go on holiday. 'I've never been to Newquay,' says Derek.

Manic comedy unfolds: Dougie is electrocuted and staggers around looking like Ken Dodd in a boiler suit and we're treated to a discussion on whether clowns ever have sex with circus chimps and then to a lurid song about the female anatomy. But running through the comedy was the show's universal sombre thread, and this episode seemed to have a darker weave than most.

Derek, the character, seems to have matured for this new series and wasn't so often seen capering for the cameras or being mocked by the loathsome Kev. Instead, there were moments when he would sigh. This was noticeable in a character whom is so often child-like, and it was unsettling to hear these grave, weary, exhalations come from the normally chirpy Derek.

He was definitely more solemn and coherent in this episode. Perhaps this is a response from Ricky Gervais who has been attacked for portraying Derek as 'special needs' and so for allegedly making fun of the disabled by placing such a character in a comedy, although I can't imagine Gervais changing as much as a semi-colon to appease the constant chorus of the offended, and rightly so. However, Derek's melancholy did elevate him beyond the 'simpleton' he is sometimes accused of being.

This more sombre characterisation was emphasised in a lengthy segment where he watches a video of a former resident. She is singing an Irish folk song and Derek sits silently, crying.

Watching the old woman reminded me of my own gran singing. She'd sit on my bed and sing me to sleep with 'Christopher Robin Is Saying His Prayers' and when she got to the line 'God bless Gran and make her good' I'd sit up and protest 'but Gran you are good!' So as I watched Derek watching the old woman I became tearful with him, realising that'll one day be me, watching some old film of my Gran singing her songs.

But that won't be today: I could go and visit her right now and ask her to sing Christopher Robin again, or Scarlet Ribbons or Baby Mine. But I don't because, instead of seeing gran, I prefer to wallow in nostalgia because these days she isn't as she used to be; she has dementia.

In the old days she was always singing and working. You'd be woken up on a Sunday morning with the clattering sound of her cleaning the metal venetian blinds and even though she loved having grandchildren around her she was forever keen to bundle us out of the house - 'away you go so I can get tidied up'. Or if my sister and I tried to have a cheeky, whispered conversation, she'd put her crochet needle down, knotted with lemon and turquoise wool, look at us over her glasses and say 'your gran's no daft.' She was just constant energy and alertness and love.

And I was her favourite, even from the day I was born when I emerged on the ward red and crimped and hideous because they'd pulled me out with forceps and the steel grips had left a big creased mark on my face which sealed one of my eyes shut. I was one-eyed, girning and grimacing and slabbering at everyone. People bent over my cot then muttered 'Jesus', but gran pushed them aside and picked me up and said I was her lassie.

But now all her humour and energy and songs are gone and she just sits quietly in her armchair and asks you the same questions, over and over. I sometimes can't even look at her as I want so much to have her back as she once was, with her black, black hair and her apron tied tight, shoo-ing us and feeding us and bathing us, stirring giant pots of orange soup called Tatty Bogle and putting her good coat on for Mass. It's awful so see her now, white-haired and silent, so I always keep busy when I visit her. I run about in the kitchen. I ask do you want a pancake and butter? Will I get you the paper? Have you got enough milk in the fridge? Will I find you a good film on Sky? I can never just sit down and lift her hand and talk because then I'll know she is away. I am a hateful coward. I will find it easier to weep over a film of her singing, after she is gone, than to visit her now when she is still here.

So Gran sits quietly in her chair whilst a new generation of great-grandchildren tumble over her feet. She looks down at them and gets their names wrong and asks the same questions. You could almost feel irritated at having to constantly say 'no, I told you I don't work there anymore. No, she had the baby last year. Yes, I made you tea, it's right there.'

But any irritation vanishes when she suddenly remembers the old days when we'd go on holiday together. She'll try to sit up straight and she'll say 'We'll get to Blackpool this year' and we all look at one another and decide to nod and say yes, yes we'll get to Blackpool this year.